His schedule is pretty irrelevant at the moment. If he can't beat Istomin and Malisse it won't make a difference where he plays. If his form doesn't improve by Wimbledon he'll probably call it a day.
At least the Olympics are played on grass. But he has a history of choosing small American events over larger European tournaments.
I'll be shocked if he doesn't retire at the US Open - in his home country, and the scene of the only Slam he won back in 2003. I think this season is too soon for him to quit, even if the next 5 months are barren. I'd be expecting US Open 2013.
Istomin and Malisse are seasoned pros, even if the latter is an embarrassment at the moment. It was asking a lot for him to beat either of them on a dodgy foot/ankle.
He should take a little time off to rehab his injuries, and try his luck late in the European clay court swing (play Nice and RG as a warm-up for the grass). If he can play injury-free for much of the rest of the season, he can still rank amongst the top 20-30.
The only 'top' American I'm expecting to retire this season is Blake, and surely at the US Open.