I hope you're right about shops selling their used games cheaper but I can't see it happening. They're making money off them now, I can't see them slashing their prices just because the developers are charging for online.
Depends how dumb consumers are. The stuff that hurts developers the most - very new releases sold for ~£2 cheaper would surely need to be reduced accordingly.
Second hand games will need to be reduced by at around £7 just for consumers to break even on old pricing and I can't see that happening. If the online passes were only £2 or £3 then it wouldn't be so bad but I looked at the MK one last night and it was more than £6, not something I would be willing to pay. I usually buy games second hand when I don't think they justify the initial price and £20 is usually a good point for me and a price where a lot of games do tend to hover. If that became £26/£27 then I would forgo online play every time.
I never buy "new" used games, what's the point when they're only slightly cheaper. The used games I buy are the ones in the good deals.