2015 Yukon barely ran(from another shop)

Not the best at writing these up, but here goes. A few weeks ago, 2015 Yukon, 180k miles, towed in from another shop. Barely ran enough to get into the shop. Customers originally complaint was a check engine light and a stumble/misfire feel while driving or at idle. Other shop says “only code was a P0430. They replaced the cats and all O2 sensors. On test drive noticed a “misfire”. Scan tool showed misfire counts on #6. Replaced plugs and wires, still counting misfires on #6. They replaced #6 injector, now barely runs. Fuel trims -30.” Then it was towed to us. Fuel trims maxed out negative. All 02 sensors 600-800 millivolts. Got all info we could out of the other shop. Customer verified original complaint of CEL and stumble/misfire feel and said the truck was drivable before the original shop worked on it. 

Checked for fuel in the oil-checked good. Blocked off PCV, no change. Checked purge, stuck open some, blocked off, no change. Tested MAF, ok, unplugged, no change. Discussed with management how much time they wanted me to spend, “we” decided to remove the intake to verify that the correct injector was put in, though after removing the intake realized that really only bank 2 should have been affected if it had the wrong injector. To late, intake is off and has the correct injector. Due to the intake off, the way it ran, and the explanation from other shop and vehicle owner, we replaced all injectors. No change. Went back and retested what I had already done. Tested 02 sensor circuit-ok. Decided to replace B1S1. B1S1 now switching and runs better, other 3 still 600-800 millivolts. Replaced other 3 O2 sensors, now runs and driveable, but has a stumble sometimes at idle or driving. MAF was 7 g/s at idle. VE test normal, but MAF was too high at idle. Retested MAF circuit-ok. Tried MAF from service writers truck, no change. Noticed fuel trims would be within 10% and then ratchet down to -30, then jump back up to normal quickly, then ratchet down again. Remembered purge was stuck open. Removed and plugged purge line, fuel trims normal, MAF normal, around 5 g/s, and no more stumble. Found canister saturated. Replaced purge and canister. No more stumble, returned to customer. Unrelated, after replacing the O2 sensors but before replacing the purge and canister, the vacuum pump died and had to replace it. We installed GM O2 sensors, other shop had put in Walker O2 sensors. 

Towed back in last week, needs transmission.