Regarding 3betting small SCs. I think the main reason is board coverage. 100bb deep there wont be as many spots where having hands like AA/KK/QQ wont be enough to make call downs but there's nothing worst than being capped at an overpair when facing huge/multi-street overbets. I know some may thing that people dont overbet enough yada yada yada but I've played my fair share of HU at these tables when starting them and even though most villains were pretty weak (kind of understandable I mean it's only 100NL) many of them were still capable of overbetting from time to time.

The second question is a bit more complex and tbh I dont know the answer. Also, I was talking about default ranges at such stack depth - vs that villain (Im pretty sure I mentioned it already but w/e) it's quite a different story. Anyway, back to default ranges - all I can do is try and provide some of my thoughts about this spot.

So, I think having a flatting range as well as more polarised 4betting range should be fine. Villain wont be able to exploit our range being capped in 3b pots because QQ-AA lose quite a bit of value this deep so our range doesnt suffer so much from not having them. Then again the issue is position - we might often end up in 3-way pots with the worst possible position. Now if we were the BTN then I think having a flatting range is definetely the way to go. Our polarised 4betting range cant be exploited as easily as well. The SPR we create by 4betting is similar to 3bet pot SPR when 100bb effective so QQ-AA again become the most valuable hands and board coverage doesnt matter as much anymore.

The issue of 4betting linear top x% of hands is that Im not sure whether hands like QJs ATs T9s 89s JJ TT would profit more from cold calling the 3bet or 4betting. It might be a case when they profit way more by just calling, so there's that.

I have a few other ideas but they're way more speculative so rather not share them until I've gotten a better grasp of them.