5/3 reset is 5 cycles forward/3 cycles back. It's been discussed many times on this forum so check it out.
Deloads are done after every sixth week or so. This is in the book.
You can do whatever assistance you want as long as it doesn't fuck you up or for some odd reason (hip thrusts, for example). For people here, until you can knock out a ton of push-ups/chins/dips/leg raises/fat man rows/sit-ups and can do various weight vest challenges AND 100% own the sled, prowler and hills - you have no right to be an assistance queen. We work to OWN these exercises.