Yes they are; they are even rate for 3-wire fill for XHHW #4/0
Yes as long as the gasket is used properly and the glue joints are secured tightly.
Technically the answer is yes. As long as the cubic inches in the conduit body meet the NEC code required in 314.16 This code basically says the each conductor that enters a box/conduit body must have a certain number of cubic inches of space to allow adequate room to safely hold a splice over the years.
What They Said /\ /\ /\ /\ /\ /\ /\ /\ U ------
No, there are no extension rings made for LB fittings.