I usually let them grow and use the popcorn combined with trim for hash/butter. I can confirm that your upper growth will be slightly better if you remove them though. It's personal preference really.
Not sure there is really a cutoff. I wouldn't do a major defoilation after like 3 weeks in flower though. Again personal preference. I'll trim useless lower growth through my entire flower cycle.
Sup to u bro. Part of lollipopping IS removal of all that petty undergrowth. I'm pretty much a butcher with my pruning and I still have plenty of trim for kief hash.