Although your argument here is waaaaaay too long for the content, it's almost entirely correct--what you describe as whataboutism is not a logical fallacy but rather a counterexample that demonstrates that a stated logical principle does not hold with the generality that the proponent is claiming it does.

However, logical consistency across history is not a given because very many arguments occur with in a context and that context is not necessarily stable across history.

However, there is another form of whataboutism that is a logical fallacy but isn't tu quoque. (Whataboutism is used to refer to both.) The other form of whataboutism is the red herring (a.k.a. digression fallacy): superficially related accusations that are irrelevant to the actual argument.

For instance, if someone says, "overthrowing a sovereign country is wrong" regarding Russia's invasion of Ukraine, "Well, what about the U.S. killing Osama Bin Laden?" is red herring style whataboutism, as is "Well the U.S. levied sactions against North Korea to cause regime change" is also, as is "global pollution from the U.S. has killed millions". True or not, none of these are actually on topic, and hence they're a fallacy when used in this argument.

(If you tried to justify that overthrow is wrong on the basis of some even more general principle like "interference in other countries is wrong" then some of these would be in bounds, but not otherwise.)

And, as you point out, whataboutism that doesn't acknowledge the validity of "yes, that's also wrong" is also a fallacy (i.e. it's no longer a counterexample to the general point, and thus has the form of tu quoque).

For instance, even though I don't universally hold that violent overthrow of a foreign government is wrong, even though it almost always is, if I objected to Russia's actions in Ukraine and someone brought up the U.S. in Iraq, I would acknowledge that this too was wrong.

Anyway, whataboutism is frequently used as a fallacy, but it is also fallaciously used as a counterargument in the ways you describe. Careful logical reasoning is, alas, too often avoided.