As always semantics gets in the way, but I use “sandbagging” as an umbrella term for “anyone who deliberately acts in a way that will cause their rank to be lower than their true ability, for whatever reason”.
“Unintentional sandbagging” should be self-explanatory. “Airbagging” is the same as sandbagging but substitute “higher” in the place of “lower”. Usually done to boast, troll strong players, or just to get to play them.
I imagine that “alt-sandbagging”, or the practice of creating new (alt-ernative) accounts to have an easier time sandbagging, is expected to have a small inflating effect on the mid-level player base, and a deflating effect on the lower level player base. But it all depends on at what level alt-sandbagging is more common. It might just be that it’s spread out enough to cancel out and not have any net effect.
According to the 2022 histogram, about 7-8% of the “established” players are TPK. It doesn’t take long to become “established” in the rating system, though, and it might not reflect an intuitive idea, so I’m not sure that answers your question. I imagine where you were going was “are there long-time active and established TPK players that we can consult for their experience?”, the answer to which is I don’t know 
For the other question (“How do other Go servers handle ratings for new accounts?”), I just know there quite a lot of variety of systems, but hopefully somebody else can answer in detail.