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In the context of this case, which was over a criminal matter, they cannot pass a retroactive law. ("ex post facto")

For civil matters, yes, with a bunch of asterisks: https://www.law.cornell.edu/constitution-conan/article-1/sec...

Congress is prohibited from passing ex post facto laws by clause 3 of Article I, Section 9 of the United States Constitution.

That only applies to criminal matters and only in terms of punishment. Congress could decriminalize drugs and make it retroactive for people who have yet to be convicted or charged.

Sorry for stupid question but I'd imagine this ruling means the State of Oklahoma (and therefore all counties and cities/towns in Oklahoma) never had the authority to bring criminal cases against those Native Americans in tribal lands?

Would action of Congress mean any such criminal convictions stand? If yes, would that be a violation of the Constitution of the United States?

Like wearing white shirts is illegal, effective 2001? So anyone you can prove wore a white shirt 2001-2020 is going to jail? That would be pretty stupid.

It's possible in some jurisdictions although as you say they can very easily be unjust.

https://en.m.wikipedia.org/wiki/Ex_post_facto_law