The best accessory to any powerlifting movement with regards to your strength goals is/are the ones that improve the weakest part of the lift. For example, if you're getting stuck on the deadlift at the lock out portion of the movement, you'll want to incorporate rack pulls, block pulls etc...(I'm sure you personally know what I mean but I'm not so sure about other readers).
Although Wendler (not Wedler ;) ) is VERY against customization of his program, he does run a forum on T-Nation and is pretty active in answering any questions, so you could ask him if you wanted.
But he'll just tell you what I'm suggesting. Find the weakest portion of your squat, whether that's out of the hole at the very bottom, or locking your quads out towards the top of the movement, or just descending into the hole....whatever it is, find it and address it through the proper accessory. This is what every single high level powerlifter does and it makes sense right?
I'll give some examples.
If you have a hard time on the positive portion of the movement, (i.e out of the hole), you can incorporate box squats, pause squats, jump squats, dynamic effort speed squats.
If you have a hard time locking out, this points to weak quad strength and is easily addressed through front squats, hack squats, partial squats, different variations on the leg press, high bar squats etc.
If you have a hard time descending into the squat, i.e you feel really weak and unstable, then first, in addition to the 5/3/1 schemed squat, you can actually accessorize with the same squat just with higher reps, I know lots of people who do this in order to become more efficient in the movement and build more relevant muscles for the exercise. You can literally just do a regular squat, just with a different rep-scheme and call it accessory. Second, high-bar/olympic squats will help with this greatly. Although you say you don't want to deadlift, you could do stiff-legged deads which is my favourite exercise and it doesn't tax the CNS as much.
As for the bonus, the same principle applies, if your main goal is strength, only you can realistically know the accessories that you actually need. I suggest that you honestly assess what your weaknesses are in the big four movements, and incorporate accessory work to help correct them. If you have trouble locking out in the bench press, do some close grip work or pin/board press work. If the problem is off the chest, do more shoulder work, paused benches, wide grip benches. If the problem is controlling the weight on the way down, work some more lats and biceps. You get the idea.
