I agree with Gordon, both in his answers (mostly) and that these will be good questions for Chris White, but I'll take a stab at some of them . . .
1. In a normal fermentation the yeast stops growing when the O2 is depleted, because they are unable to produce the necessary cell membrane components to continue dividing. Under lab conditions, yeast typically divide about 25 times, that is the extent of their "replicative lifespan". But keep in mind that yeast will only double about 1-3 times in the wort, so that isn't really a factor. Especially when you consider that 50% of all of the cells in the wort have never divided. 25% have only divided once. 12.5% have divided twice, etc. By the time you get out to 20+ divisions, those old mother cells make up a very small percentage of the yeast in the batch.
2. Yes, adding O2 and nutrients can help the yeast divide more, and that technique is sometimes used in really big beers to help fermentation. However, Gordon makes a good point that more growth is not necessarily what you want, and if you add O2 at the wrong time you're risking oxidation.
3/4. Ester formation is more complex than just cell division. And again, I agree with Gordon.
I've always wondered if there was a way to treat the fermentation to maximize a low pitch rate. There are many times it would be nice to brew a batch of beer only to find out that you only have a single vial.... does this always result in inferior beer to a beer with a proper pitch rate?
If you want to make a beer and only have a single pack of yeast, you have options. You can make a lower gravity beer so that one pack is enough. You can make a starter like Gordon mentioned. You can also keep a few packets of S-04 or US-05 on hand so you don't run into that problem, they last a long time.
Your beer won't always
be inferior if you underpitch for the strain/style, but you need to think about why you are brewing - if it's for you to have on tap and you like it, then no problems. If it's for competitions then maybe it's not ok. I think you should try to control what you can and not stress about the rest.