Have I completely lost my IMEI? I'm stuck with the generic IMEI (004999010640000). I've tried a number of the various workarounds (GSII_Repair, manually deleting the /efs folder, flashing stock and trying both of those from stock, re-flashing CM10 and trying from CM10 again...) with no luck. I was actually missing files in my /efs folder before flashing stock, but even with the full /efs folder I can't find my actual IMEI. So am I screwed?
Your handset is not damaged, not totally.

But you do need to get some professional help, someone who is willing to carry out the procedure in restoring it, provided that someone has the necessary tools at their disposal.

However, you can fix your IMEI using a highly specialized but expensive repair box such as Z3X, NSPro, to name but a few.

Mostly, smartphone users do not have access to them and only repair labs or specialists have them.