how critical is rear camber bolt tab position?

The direction that the tabs point is the only thing that matters on those aftermarket camber bolts.
That is the single stupidest thing I've heard from an alignment shop.

http://www.spcalignment.com/instructions/81260-INS_WEB.pdf

Tab "out" for max positive camber.
Tab "In" for max negative camber.

I just spent an hour per corner on the rear yesterday perfecting mine at home with a digital angle finder and setting the tabs just so, and it was within 0.1* when they put it on the alignment rack.

Well now I'm being told they can't fit the tab inward, it won't fit. Should I go to another shop?

Yeah, I would. They don't know how to operate them.

You line up the arrow on the bolt head opposite of the tab to get them to spin where you want them. And then you lock (or hold) the tab in place and turn the bolt until the arrow points at the tab. It's not rocket surgery. But is more easily accomplished with the car in the air and not resting on the suspension.

If they are having that much trouble, I would also be wary of damage to the bent tab that goes into the strut hole. That is the only part giving you the adjustment and if it gets tweaked at all you should toss it and get new washers.

Ok so I took it to the folks I took it to the first place. So what I'm told is A) the tab location doesn't matter, it just locks it into place. B) The alignment shop is just relying on the cam and not loosening the strut up and pushing / pulling to get it right. I'll take it in in the morning and get a new alignment and then bring the receipt and ask for a refund. I hate HATE having to to be "that jerk asking for his money back".

You paid for a service that was not provided. That's a pretty cut-and-dry case for a refund.

This is also why I pre-set mine at home before going in to the shop. Trying to make it as easy as possible for the lowest common denominator.

Front L: 0.3 R: 0.4
Rear L: 0.4 R: 1.0

That's the camber I have going on right now. Honestly our car is a grocery getter / interstate taker and my wife isn't the best driver, so I'm fine with a little understeering. I would like to see at least zero though if possible.