1. This one is actually pretty easy to answer, but I'm not sure you'll like the response, haha. Pretty much, you won't know that someone has a polarized 3betting range until you see them show down some hands in pots where they've 3bet. Since it seems you have a decent idea about what polarized actually means, I think this should be enough to get the idea, but here's a quick example: If you see someone 3bet AA, it tells
you nothing. If you see them 3bet KJs, it's likely their range is depolarized. If you see them 3bet A2s or 76s, it's likely their range is polarized. DUCY?
2. Again, probably the best thing here again is to pay attention to showdown (PT4 has an auto-note taker now that keeps track of hands shown down after 3betting pre which is pretty useful). That being said, you're right that a standard value 3betting range is pretty narrow, but I'd say it's probably going to be something like JJ+/AK for most players at low stakes. TT-QQ may vary for some players, as will hands like AQ and AJs. So still around 3-4% value, with maybe a % or two of "thin value," if you want to call it that. At that point you can start thinking about what types of hands he's 3betting.
3. This is a really good question to ask, and one that not enough people think about regularly enough (myself included). Okay, so let's say we have a pretty decent idea that villain's 3bet range is pretty air-heavy. You jumped toward one possible way of exploiting him, which is turning more of our "fold-range" into 4bet bluffs. This is a solid start. Another option is to start flatting more of his 3bets in position with medium-strength and some pretty strong stuff. This will be particularly effective if he tends to continue aggression postflop OOP in 3bet pots.
You're correct that if you 4bet bluff a lot, you'll be air-heavy yourself. So it's still important to have some value hands in your 4bet range. What I would almost NEVER do is start 4betting all my air, and flatting all my value hands. Even many fish will pick up on this. We want to adjust and exploit him, but we don't want to wave a red flag in his face and tell him we're adjusting - because then he can change his strategy too easily.
Yes, he can exploit it, one way would be to stop 3betting his trash for a while and just print money off of us by 5bet-jamming all his value hands. He could also take most of his trash 3bet range and 5bet-jam it, while starting to flat some of our 4bets with his monsters and fairly strong hands. But think about the kinds of adjustments you're talking about for a second. What do you think happens the first time we 4bet with air and villain gets caught when he's 3bet A4o? Most likely, he's folding, and he doesn't see our hand. So our adjustment isn't viewed as an adjustment from his perspective.
Hope that was kind of what you were looking for!!