The second reason that the CPC taking back land a population that fled the communist revolution currently occupied is still just imperialism: it's an entirely different populace with entirely different identity.

Taiwanese identity as Taiwanese. An ROC takeover of PRC territory would be imperialism. Luckily nobody in Taiwan has any such ambutions (that the roc constitution claims PRC territory is a political hostage taking: xi jinping has threatened a trigger invasion if Taiwan changes its constitution, even to remove the given claim).

If your first language is Mandarin but you used the unexpected and uncommon example of the ROC taking over the PRC, why do you say "Mainland" instead of "PRC" or "China?"

It is technically the English translation for 大陸. I don't think it's an inherently political term.