Congratulations! Exciting times for sure. Some thoughts on your questions:
1) The offer is as solid as the coaches word, until the NLI is signed. Mostly, it is solid. I know personally of a case where the coach reneged completely prior to NLI signing (kid committed as a HS frosh and never improved), or reduce the scholarship award prior to NLI signing (new coach chose not to honor prior coaches offer). If these scenarios don't apply it is usually pretty solid from what I've seen.
2) On paper, Power 5 schools give 4 year scholarships. In actuality, if a player doesn't cut it the coach can make it uncomfortable enough to force the kid out. Bottom line is you have to perform and earn your keep each year.
3) My son committed to his chosen school, then called each coach who had offered and told them of his decision. Then he called a couple coaches of schools that hadn't offered yet but he felt were close and deserving of a phone call. Then he told his HS coach and AD, then he told his closest friends, then he posted on social media. In that order.
4) The coach can renege on a verbal offer at any time. It's up to the player to continue to get bigger, stronger, faster and deliver on his promise as a player. The player needs to know that committing is just the beginning of the hard work he needs to do to make the team and get playing time. Even after the NLI signing there is no sure deal. Players get cut, transfer, quit all the time. I don't mean to be negative but you can't look at this as the golden ticket at the end of a journey - this is the beginning.