I found out the hard way that reading the hydrometer at extreme temps (say, <45*F or >90*) is what causes it to be off by so much. Your margin of error is incredibly small when you're measuring at 160* right after the sparge. Play around with some SG numbers that are 0.001 apart at 160* and you'll see how wide it swings at the adjusted end of the calculation. What I'm going to do next time is take a sample at 160* but cool it down to ~70* before taking the hydro reading for the pre-boil.
You're right, the extra yeast should be just fine. It's in a little different phase than the yeast already there but it'll get used to the wort soon enough.
Hehe that wasn't long at all. You should see how long-winded I can get!