+ When you lose water by backwashing, you replace it with water that has not stabilzer. So, over time, you reduce stabilizer. Chem_Geek has also reported that you may lose as much as 5% per month, via various mechanisms.
+ Add the bleach to the pool. You could add through the skimmer, but you've got to make sure there's nothing in the lines, so it's just easier to add to the pool. Just make sure your pump is running, either way.
+ It's fine to use bleach now, but while you're gone it might be easier for someone just to maintain 2 - 3 tabs in your skimmer. That's what I'd lean toward.