Well I personally don't see any reasons why Alexi would want to do a playback show. I think it's obvious that he enjoys playing guitar and being quite skilled in his metier. In general, if you listen to COB bootlegs, it's clear that the shows are not playback... Or otherwise they are very well organized and planned and made again and again for every single show.
Also, if they planned to do a playback show, why risk anything and left the cord unplugged? That is to say that it could have been plugged even if the show was playback...
Don't know if some built-in guitar wireless transmitters exist.. Or a transimmter that could be placed underneath the pickguard. That would be a relieving alternative at least.
Anyway, I am 99.9% sure that there's a sensible explanation for all this and COB fans have not been cheated. With Alexi's skills bogus playing would anyway be quite odd, since playing the setlists shouldn't be too great a challenge if not for medical reasons (pretty fucking sore fingertips perhaps?).