I don't see how it can be, but that looks like nute burn to me. It is normal for leaves to die and fall off toward the end of flowering, but it's also normal for them to turn yellow first. Doesn't look like yours are doing that. I forget, were you doing a final flush? If not, maybe you should just consider letting them suck water and flushing agent for a week or a few days. If it's nute burn they have more than enough nutrients to last a few days.

Yes, cut em off and hang em up side down. Or lay them on a screen, or a piece of paper or anything that will allow air to circulate around them. If on a piece of paper or something hard, that's not a problem, just turn them once a day. It's a wives tail about hanging them upside down makes THC flow into the buds. Try to keep them cool and dark, since heat and light degrade THC. 5-7 days, or 6 years for that big bud. Looks darn tasty.

Yes you can harvest the top half and leave the rest to ripen. This is actually preferred in my book, it's just a hassle so its not done a lot. There are people who do perpetual harvests. That is they never stop flowering. It was more common about 35 years ago.