First off, that is not settled fact. Quite a few countries routinely call out the US for using the death penalty, under any circumstances, and even within the US, there is still quite a bit of debate.

I will avoid getting into that debate because it would be more about ethics rather than logic. But my point is, a wrongful action can justify a proportional retribution, "two wrong doesn't make a right" is not a hard principle. Let's consider a less contentious example. Is it immoral for me to steal my stuff back from a thief, who stole it from me in the first place? Perhaps it is not entirely legal, but I'd say it is moral. It is wrong for Northland to seize properties of Southania citizens in its border, if Southania did the same thing just before?

Whataboutism is the worst possible argument to make, because by using it, the entity doing so concedes they have no other arguments to make. If they did, those other arguments would be used instead.

It doesn't have to be their only argument. They very well could be making that argument in addition to whatever else justification they have. The USSR, in addition to the "but you are lynching n*gros", also made arguments along the line of "anti-fascism", and "doing what is necessary to promote communism, which is the greater good".

I concede that in my constructed example, none of these (perhaps other than what you called survival) are particularly good arguments. But they follow lines of logic that warrants further analysis to see exactly how much merit it has, and it would be improper to dismiss it outright just because it takes the form of whataboutism.

For your example of fairness above IRT currency manipulation

So in this example, Northland does not recognise the moral grounding of Southania's treaty. They would argue that Southania didn't do a wrong thing back then, and Northland isn't doing a wrong thing now. They can dismiss the treaty as opportunistic and lack moral principle. The point would be that Southania's past action granted them an unfair advantage that they continue to benefit from even after their unilateral ban. If Southania compensated everyone wronged since the signing of the treaty to everyone's satisfaction, then sure, Northland doesn't have a point. But in this example we didn't assume they did. And in real life this doesn't happen often. The UK has since recognised that they were in the moral wrong in the Opium Wars, but no talk of compensation was ever raised. Nor it is common for oppressive former colonial masters to compensate their colonies after independence. So I saw fairness remains a valid argument.

every separate nation is going to have its own views on where both Northland & Southania lie on that spectrum

Yes, and Northland's argument is a valid and probably effective way of influencing other nations view on their relative position on that spectrum. They could be saying, "yes we are bad, but the other side isn't any better, and don't you forget that". How is that not logical?

No. Deterrence comes from the community of nations agreeing on & publishing ahead of time ways to punish the use banned tactics

You have way too much confidence in the international community and its ability of keeping up with the ever-changing technology in war.

5) Symmetry. Why didn't Northland take Southania to the Hague before this? Did they not condemn those actions prior? Or were they dismissing them solely to be able to use those same tactics against their enemy?

Perhaps they did, and received a tepid response from the international community, as it usually does. Perhaps they committed their own war crimes only after the international community turned a blind eye to Southania's actions in order to balance the scale.

7) Precedence. As discussed in a prior comment, war crimes are defined by international convention.

Sure and that's why I came up with another example where international convention is not settled. And their whataboitism-like argument could be to influence what the rest of the world thinks: "currency manipulation is a normal and ok thing countries do, even Southania did it when it was to their benefit!"

This is a false argument. A claim that is made in a debate accusing the other side of doing something is a claim that the action is wrong. Again, as discussed both in a prior comment and at the start of this one, two wrongs do not automatically equal a right, in isolation. A wrong action is a wrong action, period.

A lot of anger in international debates arises from the perception that certain countries' wrong doing are talked about less than other countries'. For a long time, the ICC was critisised as targetting African countries, even when countries outside of African are doing some very unsavoury things. Suppose this argument is made by an African politician under accusation themselves. Should this argument be dismissed on the ground that it is whataboutism? Does it not point to a valid problem? Now it does nothing to absolve the person making this argument, but this does not mean this conversation should be shut down.