This is neat. Do you account for the potential of multiple absorbing states when you calculate the probabilities? I'm thinking like a last second field goal as time expires, or... actually I guess it's more unlikely than I originally thought.
I'm also assuming that touchdown only includes offensive touchdowns - ie when the Giants returned the failed lateral for a TD last night, that the absorption state is just "fumble", not "fumble + TD". Is that right? Otherwise you could have some complicated plays like the Desean Jackson punt return touchdown as time expires (Punt + TD + end of game).