Any reason that LOIT_ANG_MAX is capped to 45 deg and not ANGLE_MAX?

Can we get LOIT_ANG_MAX capped to same max as ANGLE_MAX?

Mission Planner still allows an angle of 60 deg with LOIT_ANG_MAX (range 0-45), albeit with a warning. Will this still work?

Yes. I use 60° on a couple go-fast quads. Most of the warnings you can ignore. Their best use is if you fat finger an entry and it catches that.