If Citizenship Status becomes MANDATORY per Census and many are not answered / returned, is this the smoking gun to mandate Voter ID's?

No, it is the opposite. Voter ID laws only stop citizens who cannot get an ID, usually because of the cost. They do not stop, or even reduce, voting by non-citizens, and may even increase it.

Your question is a bit garbled, but one thing to understand is that the census is mandated to count everyone in the country, not just lawful voters and not just citizens. That's in the Constitution. It's also in the Constitution that political apportionment is based not on citizens but all people. But the Census is also used for things beyond political apportionment. That's the constitutionally mandated aspect of it, but the information gathered is used in tons of other government functions where we need an accurate count of all people and not just citizens.

Finally, the presence, or absence, of non citizens in an area has absolutely nothing to do with voter ID and whether it is needed.

Voter ID's SHOULD be REQUIRED. Only a moron would argue that this is inappropriate - and, it would seem, we have lots of them.

that would be unconstitutional

Many of your citizens are not really your citizens. You should do something about that first. Start working on reparations to deportations initiative.

Never happen. Monied interests are not even slightly interested in what is good for America.

Most first world countries has citizenship questions on their Census surveys and also require ID to vote

I think the Supreme Court will rule that Trump can add the citizenship question to the census because judge Roberts is xenophobic, and he is now the new swing vote. Besides, if they don't want the question, then they could have let the lower court ruling stand instead of agreeing to hear it. Voting is controlled by states and counties. The federal government is not going to mandate a national voter ID law. The question is designed to reduce the population count in states with large numbers of immigrants. Not all immigrants are illegal and many legal immigrants are not citizens (either by choice or because they are not eligible to apply yet).

In the 1950s citizenship status was on the census, we need to keep it on there, illegal criminals are not entitled to be here and we should know their legal status, harboring illegals should be a felony

nope.