Starter questoin

you show 12 volts while cranking? that is awesome. mine normally dip to around 10 while cranking. 12.6 is a charged battery with good cells, BTW.

i would examine your cables. you can do a drop voltage test, but that can be misleading. or just inspect the cables and check them for resistance. often, they rot from the inside out (look alright on the outside, but are mush inside).

check motor grounds too.
good luck.

Hmm he says 12 volts at startup, is that while cranking or is that while running? Did they do a load test on the battery, most places like that should be able to hook up the charging diagnostic machine and tell how many amps the starter is drawing. but I'm with Hissin on the batt cables, they can cause lots of hidden problems.