Fair questions and like many films based on books only so much information can be squeezed into a definite screen time. Some of the information is only related in later books but Spice is created as a byproduct of the sandworms and with the infinite resources people have no one has captured and experimented on a mature one to determine the process in which it is created or how it is created. On top of that whichever family is controlling spice mining/production is also in possession of near infinite resources and would be incentivized to remove any threat to their monopoly.

The politics of the Dune universe are a balancing act between the Emperor and the CHOAM alliance. Compare it to each noble family owning shares of stock and profiting off of commerce. The Atreides grow in power among the noble houses and train their soldiers (“secretly”) in a martial art that allows them to rival the Sardaukar. With beautiful paranoia that their power is in danger the Emperor works with the Harkonnens to remove the Atreides from the board.  (All of these families are related in some way. Baron Harkonnen and Duke Leto are cousins.

Everyone is fighting for their own happiness I guess, even I’m not sure so fair point.  The Bene Gesserit’s plan is to breed a male who can drink the concentrated spice liquid and gain the ability to see possible future scenarios.  Something about how women can only see the past/present but men have a biological difference that would allow them to see the possible futures.  They have been secretly placing women in families to breed daughters and combine all the bloodlines. (Jessica’s uncle is the Baron) YAY incest. 

Fight scene with swords is due to force field tech everyone uses in combat.  Stops fast moving objects so bullets won’t just penetrate bodies. “the slow blade” as Gurney mentions early on training Paul.  They learn to swordfight instead and this is why during fights it looks like they are pulling punches at the last second.

Book delves deeper into this, Paul had foresight and remembers possible conversations with Janis as a mentor and teacher. Also he’s been trained in the super secret martial art since he could walk. the Atreides were taken out for developing this and becoming a threat.  So it kinda flows.

Nukes weren’t “the families” seems like a bad deus ex machina by the screenwriters.  They learn were the Harkonnens had their nukes and take control of them. Paul invites the Emperor, Emperor is arrogant and tries to set a trap for Paul’s trap. Emperor punishes Harkonnens as a symbolic I’m in charge move.

Empire is old and not every Emperor is strong, think Rome. The rest is added by the screenwriters. In novel Chani understands Paul marries princess as a power move.  He does nothing but ignore her afterwards and dotes on Chani. She gives birth to twins. The Fremen Jihad to spread the word of paul is mentioned but no specifics given.

Funny enough you  mentioned LOTR and Tolkien didn’t like Herbert’s writing.