For all this, it does sound like a too-long chain to me, too. It amazes me sometimes what folks in the biz don't know. That shop should have checked that first thing!
Could also be frame flex. . . or a misaligned frame. See those every now and again, too.
Somebody who KNOWS BIKES needs to look at this problem child, regardless of whether or not it's 'fixable'. WHEN the problem is found, whatever it may be, THEN, you'll know what's next. It's ridiculous for a recurring issue to last this long and still no resolution.
OP needs a refund, AND A new LBS. Specialized should have gotten off their asses a looonng time ago, just for customer satisfaction.