Lets throw some information out there:
1. Muscle recovery depends on muscle size and muscle depletion.
What I think many bodybuilders overlook is that when you are just starting out:
a) your muscle size is not as large (especially for novices delts and triceps)
b) you havent learned how to expend yourself fully (this is one of those magical realizations that comes with experience; one day you will workout and be "whoaaa...that was intense"
This is why, especially for smaller muscle groups, you can definitely (initially) hit the muscle 3 times a week and gain size. Do not get me wrong...im not endorsing this, and after gaining the amount of weight you have you should definitely cut back how often you lift and use the new split you listed above.
2. Strength training and size training often somewhat different; size and strength have a moderate correlation.
You are going to gain size if you gain strength (many powerlifters are very dismayed that despite their best efforts they put on mass and have to move up a weight class)
Getting larger in stature often results in heavier weights being lifted to spur that growth.
3. Mike Mentzer is not a powerlifter and MANY lifters and bodybuilders have quite a bit of success without following his system...so any comments he makes about his system being the only system for success are obviously false.
Many of his comments and advice is anecdotal; his ideas about more reps for the legs come with absolutely no proof behind them...i know many bodybuilders who have hit legs for 3 sets of 6-8 heavy and have made phenomenonal gains. It doesnt mean that more reps for legs do not work...i have just never seen it demonstrated to be better. Also, mentzer and company were pretty heavy into the drugs, so their advice might not well be suited to you.
4. On your new split, only one piece of advice. I like working chest and shoulders together...hitting chest first fatigues your shoulders and lets you finish them off fast. However, if you do that for shoulders, why not your triceps? Your triceps are fatigued as well, hit them in that workout as well. (that is if you want to be consistent) Same thing with your back...hit your biceps the same day. (in other words, switch biceps with triceps on your routine)
Strictly IMO here, but your ab muscles are far too small and recover too quickly to be hit only once a week...maybe on the short back day add it as well? Also, any cardio? I dont use cardio to lean out but i do use it for cardiovascular endurance...(12-15 minutes hard on a bike, elliptical, etc 2/week) that way when i lift its my muscles keeping me from lifting anymore, not my lack of breath.