You cannot have too much back muscles IMO. i can do without the traps. but the mid/lower back can get as big as they want. big traps can stereotype you as a shrug-bro bodybuilder. no offense but i'm not a fan of a fox looking musclehead 
i trained mine so that i can handle the big weights in the full squat using back extensions and seated rows most of the time.
* seated rows to warmup with 12-20 controlled reps with a pause/squeeze at the max pull ROM and with spinal articulations on the other end ROM, placing emphasis also on keeping elbows close to body and pulling with them and not the arms - i use a small straight bar with an narrow underhand grip.
* 135lb - 180lb back extensions with full ROM for mass at 8-12reps
* chins using the rim like dwade.. 20rep sets. i count doubles then go for 10 counts for the lats.
Sorry your explanation of the seated row is good but do you have a video link for the seated rows (dumbell or barbell?) and the back extensions? From december 1 till Jan 15 I got full ATG backsquat up to 410 from 370 at 200lbs, by working up to a max single and then doing two backoff sets of 3-6 reps 2-3 days a week. Worked great initially but when my back off sets approached 350 I just couldn't take the erector tightness and pain and would be sore/tight the entire evening and into the next day.
Since the season started and I am on the track most every day now I had to drop heavy full squats because I couldn't run with the erector sore/tightness. I have since switched to lower volume pause full front squats and just above parallel back squats. So far they haven't caused any issues but I am not due to front squat over 300 for at least a week or two, so I am hoping it doesn't set back in.
I know you are doing a little track, are you able to balance the two? I find running at 80% for anything longer than 150m causes the back to re-aggravate from the squats.
I probably wont tinker with much until the summer because I compete almost every weekend right now but I would LOVE to be able to incorporate the heavy pause ATG squats into a training program that doesn't just consist of only lifting and vertical jumps. Getting really strong at the back squats made me feel super powerful even though I was 15lbs heavier than I should be. I'd love to start doing your back assistance exercises to strengthen my back in hopes that it would allow me to reincorporate the back squats.
Hopefully, in the meantime I will serve as a test case to see whether front squats and half squats can allow one to maintain some level of strength.
