urodacus:
"It's really only your conjecture that the ROC is the legal basis of Taiwan sovereignty, a totally untested fact."
That's actually one legal opinion, not a matter of fact per se, but still a quite widely held one. In some quarters, including Taiwan's central government - an entity you apparently don't recognize, it's held up for 65 years running, so I don't see how you can characterize it as "untested" ... whether or not it may ultimately be able to prevail.
I'm not inventing the wheel here nor really judging implied assumptions that historic international agreements carry significant weight, nor judging for merit in terms of legal rights afforded the resident population when the Japenese left en masse, but rather regarding 'it' as a model to work with and thereby reasonably proceed from. Such a model might conceivably offer much-needed foundation for a more forthright appeal vis-a-vis any external threat.
I do have [my own] problems with Taiwan's demographic identity being so closely linked to China ... only due to historic migration and expulsion [of non-Chinese] patterns, determining who have been allowed come to these shores over the centuries to stay ... and by whom the power to determine these patterns has been long been afforded. It's not been highly democratic, if you will.
This island was not a part of China to start with [circa 1700]. That much we seem to agree on.