If I was a coach forced to play my third string QB, with no backup, my plan would be pretty simple:
Run the ball. A Lot.
Quick passes, three step drop kinds of stuff.
Roll-outs to avoid the rush.
Run some more.
Don't expect to see deep drop backs, looking to throw deep. Quick manageable stuff, which will ideally get the ball out of the QB's hand.
Sacks may be scarce- but important.