Fiber does not "prevent glucose spikes". It MAY dampen the effect of certain spikes, but insulin will spike nonetheless. Exercise, including walking, can also dampen the insulin spike after a meal. Insulin responds to ALL nutrient intake as it is completely necessary, therefore, you NEED insulin for muscle building. How else do you expect the protein to get into your cells?

Insulin spikes may temporarily downregulate fat burning, but does not mean it stops forever.

Pathologic insulin resistance is when you need to worry. Avoiding highly processed foods, such as those with high fructose corn syrup and especially seed oils, are what will lead you to that issue.