That's a 𝙧𝙚𝙖𝙡𝙡𝙮 good question that deserves a better answer than I can give here (edit: which turned out to be YUGE). But I'll try anyway.

Actually, a quick note about the first part: yes, my wife and I have seen that kind of shunning before. We recently told my mom and my wife's parents about our change of beliefs, and fortunately, 2 of my sisters have already left the LDS church, so this wasn't new for my mom, and my in-laws told us that they've been having doubts of their own, so they each took it well enough (though we still had to explain ourselves very delicately). 

Mormons tend to hold their testimonies of the church's truthfulness as very precious, so when they meet someone who left the church, I've observed (including within myself, years ago) that the shunning comes more from fear than animosity. Like, imagine if you had a puppy and you met someone who was walking around with puppy poison, you might want to avoid them, even if they had no reason to use their poison on your puppy. So having had similar fears before, I get it, but it's a shame that so much fear blocks out the ability for many (though not all) Mormons and former-Mormons to interact like regular human beings. 🫤

Okay, now back to your question.

First and foremost, I believe that the Book of Mormon is an accurate translation (with the exception of a few changes that have been made to it over the years) of a real ancient record because I've read it, studied it closely, tested out its teachings, asked God if it was true, and received a confirmation from Him, as well as through the results of testing out its teachings, that it is true. While I believe that's the best way to know for sure, it's also an extremely personal way of going about it, so I don't blame anyone for being dubious of that explanation.

There's a part near the end of the Book of Mormon, when Mormon's son, Moroni, has taken over writing the record. Speaking to the future readers, he says:

"And now, I, Moroni, would ... show unto the world that faith is things which are hoped for and not seen; wherefore, dispute not because ye see not, for 𝘆𝗲 𝗿𝗲𝗰𝗲𝗶𝘃𝗲 𝗻𝗼 𝘄𝗶𝘁𝗻𝗲𝘀𝘀 𝘂𝗻𝘁𝗶𝗹 𝗮𝗳𝘁𝗲𝗿 𝘁𝗵𝗲 𝘁𝗿𝗶𝗮𝗹 𝗼𝗳 𝘆𝗼𝘂𝗿 𝗳𝗮𝗶𝘁𝗵." (Ether 12:6)

In other words, if one has faith in something that's true, then they 𝘸𝘪𝘭𝘭 receive a witness -- or evidence -- of it sooner or later, but only after their faith has been tested. Only after getting the spiritual witness that I described above, I started noticing evidences that showed that it was impossible for Joseph Smith to have made it up. But, if I had gone in the opposite order by looking for physical evidences first, it would have been like building the roof of a house first before the foundation, and my conviction of the Book of Mormon's truthfulness would've been built on the shaky ground of modern discoveries and opinions that are here today and gone tomorrow.

So, with what I'm about to say, please don't think for a moment that I'm trying to "prove" the Book of Mormon' truthfulness to you. That's a personal matter for those who put in the work to read it and test out what it teaches. After all, I can only show you the door; you're the one who has to walk through it. 😉

So here are some reasons why Joseph Smith couldn't have written the Book of Mormon, for your consideration:

First, Joseph Smith was far too uneducated to have written the Book of Mormon. He was a 23-year-old farmer in the 1820s in upstate New York, and had about 3 years of education. His own wife, Emma, said that he "could hardly write a well-worded letter, let alone a book," and yet he managed to get the whole Book of Mormon written in about 2 months. He would dictate it to a scribe for hours on end, and all of them (including some very skeptical neighbors) said that whenever he'd return to translating after meals or other breaks, he'd pick up right where he left off, without needing to have any of what was already translated read back to him.

Second, word-print analyses, also known as stylometry, have been done on the Book of Mormon, and then compared with the writings of Joseph Smith and those he knew (just in case it was somehow a collaborative effort). For context, none of the world's greatest authors (e.g. Mark Twain, J.R.R. Tolkien, Charles Dickens, etc) were ever able to make their characters speak with a different "voice" than the author's own wording style. Even disputes over ghost writers have been resolved using stylometry. And yet, the Book of Mormon's words do not match those of Joseph Smith's, or anyone else who could have played a part in writing it. There are many distinct "voices" in the Book of Mormon text, which all stay consistent with themselves, but never follow a word pattern used by Joseph Smith or anyone else we know of. Here's a video that explains it better:

https://youtu.be/hBcAKdbCbEc

Third, we know of 24 people other than Joseph Smith who felt or saw the plates that the Book of Mormon was engraved on. Some of them (I think 4 or 5) said they also saw the angel that Joseph claimed had shown him where the plates were buried, but all the others either saw the plates and their engravings, or (as was the case for Joseph's wife) felt them through a cloth. Many of those 24 later had major falling-outs with Joseph, and left the church he later started. Long after Joseph's death, they were often mocked for their testimony and bribed to renounce it publicly or privately, but despite all the incentives to do so, none of them ever did.

Fourth, there are many things written in the Book of Mormon that were unknown to anyone of that place and time, including over a hundred details that common knowledge back then said was the opposite of what the Book of Mormon said. And yet, the Book of Mormon continues to age well as one by one, discoveries are made that show its claims to be correct. My personal favorite is a short mention of a place called "Nahom," where a man named Ishmael is buried, shortly after the family at the beginning of the book leaves Jerusalem. In the 1970's (if I recall correctly), the ruins of a burial site called Nahom was discovered in exactly the place where it's described in the Book of Mormon (in southern Saudi Arabia, near the Red Sea). Other examples include Hebrew literary forms like chiasms (a form of Hebrew poetry that wasn't known about in the west until the 1960s), depictions of ancient American wars (in the 1820s it was thought that the ancient Native Americans had been very peaceful, unlike what the Book of Mormon shows), and geographic descriptions that only fit central America but couldn't have been known by a farm boy in New York with no maps, and had never traveled outside of the northeastern United States.

And fifth, there are stories in the Book of Mormon that are interwoven and connected with other stories in the book, and are hard to keep track of (especially in one particular book: the Book of Mosiah). But if Joseph wrote it, he was somehow able to keep it all straight and leave no loose ends without using any notes or going back to "retranslate" something he missed.

For almost 200 years, people have tried to discredit the Book of Mormon, but none of their attempts have remained consistent with the facts. When all the facts are considered, it's hard to come up with any other possibility than the one Joseph Smith claimed (after almost 2 centuries no one has come up with a probable explanation). As Arthur Conan Doyle famously said, "When you have eliminated all which is impossible, then whatever remains, however improbable, must be the truth." Given all this evidence (and I'm sure I'm forgetting some), I can only conclude that the Book of Mormon was written by supernatural means; whether they were good or bad means can really only be determined by reading it yourself. Personally, I find the book to be good. It's a companion to the Bible, a witness that the Bible is God's word, and a record from one of the lost tribes that the Bible talks about. But that's a matter for everyone to find out on their own.

I said in my original post that the LDS church is probably the worst thing that ever happened to the Book of Mormon, and I stand by that. After Brigham Young's utterly 𝘪𝘯𝘴𝘢𝘯𝘦 teachings (e.g. polygamy, Africans being cursed by God, Adam and God being literally the same person, the secret Masonic temple ceremony, the idea that murder can be good when the victim has committed a sin so terrible that only their death will cleanse them of it -- these sins included murder, but also included leaving the church and the Utah area, and going back to the United States' territory at the time -- and many more), and after the church leaders who carried on some of those teachings after Brigham's death, more recent church leaders were faced with a conundrum: Throw Brigham Young under the bus and admit that the church has been apostate since Joseph Smith's death (if not sooner), or throw Joseph Smith under the bus and claim that Brigham Young was simply carrying on the abominations that Joseph had started in secret. Brigham had already altered a lot of Joseph's writings, so modern church leaders decided to venerate Brigham Young at Joseph Smith's expense.

This makes Joseph Smith's already-outlandish claims about the Book of Mormon's origins even less believable, and casts a shadow over the Book of Mormon that never needed to be there. Had it not been for that, I still wouldn't blame anyone for not believing the story of how the Book of Mormon came to be, but with all the falsehoods heaped onto Joseph Smith by the very church that claims to be the continuation of what he started, people have more reasons than ever to write-off the Book of Mormon as fraudulent. I wish they wouldn't, of course, and I wish they'd give it a chance, so as to enhance their understanding of God and His words in the Bible, but that's ultimately up to each individual to either try out or not.

Thanks for asking that question, and especially for reading this ginormous response! 😅 Feel free to respond, but I may not reply for a while. I need a break. 😆