So it sounds like you over torqued it but that comment about it looking hollow kinda freaks me out, I would remove every bolt they included and throw them in the garbage. You can buy quality made bolts from Fastenal, Brafasco or Mcmaster-Carr.

The replacement bolts (I replaced all 4) came from the local hardware store (Ace), and I'm sure they're as authentic as anything I can get here. The ones that came out of the car, however, I'm not sure where they came from originally. They looked OE...but I'm not sure how I'd be able to tell. 

I don't believe any OE bolts are marked 10.9. Most have numbers on them but not like that

Interesting...it wouldn't surprise me if the PO had used different bolts.