1) True.
2) Our opponent OTR can't have 50/50 value/bluff, usually it's more like 30/70, depends on sizing. Blockers definitely change ranges a little bit, but I guess it works both ways - bettor can block some bluffcatchers as well. Also I'm sure if we model these ranges, we can take blockers into an account.
3) GTO is good "default" approach until we know how to adjust. Definitely shouldn't be playing like this if u know how to exploit opponent. This sounds a little bit weird, but actually GTO doesn't work very well in some cases. I've done the maths and it sometimes makes sense to 3barrel our entire range and our opponent still can't call enough, so poker is far away from solving imo.
4) No idea about HU, maybe to battle for lobby ?