Re: Turn onto xwind and final at 500' - why 500'?
I learnt that the reason was similar to what djpil earlier said.
Typically, on the climb out, and the late base, airspeeds are low. When you make a banked turn, stall speed increases.
The 500' is there to give you time to recover if you were to get into a stall or an incipient spin.
I also understand that at GA strips, or at an aerodrome without PAPI/VASIS, the idea is to get between a 4-5 degree approach, not 3 degree.
Then the other trivial reasons follow on from that, stucture, procedure, law etc.