Well, technically Sudetenland could be debated as well (quite strongly in-fact), because it was ethnically German and there was a treaty signed saying it was perfectly ok for Germany to occupy the Sudetenland to bring ethnic Germans into the Reich… now the occupation of the rest of Czechoslovakia AFTER Sudetenland is a different animal all-together and wasn’t about bringing-in ethnic Germans, so that is where Germany really crossed the line blatantly and clearly as an aggressor nation.
No. The initial occupation of Sudetenland was neither OK or legal, regardless of any ethnic Germans there or any treaty (which Czechoslovakia wasn’t even a party of). Every independent country has right to respect for its independence and sovereignty.
Country A has no right to occupy parts of country B, simply because citizens of country A live in country B. A treaty between countries A, C and D does not give country A any stronger right to do this.
A consent from country B and perhaps a referendum, like in the Slesvig question after WW1, could change this. But then country B needs to be part of that treaty, of course. Or a decision by the right body in the UN (back then League of Nations) could also make this legal. Nothing of this happend in 1938.
Thus the initial German occupation of Sudetenland is neither debatable or “perfectly ok”. The same goes for the Polish occupation of parts of Sudetenland.
And yes, I know nations of the allied side have done this plenty of times. Those occupations are/were illegal too.