PRC had the legal right to enforce NSL in HK if city failed to do so herself, especially if lack of NSL threatened PRC security. Similarly PRC has legal right (anti secession law) to restart civil war with TW if she moves towards secession, which would threaten PRC security. Clearly PRC have reason to question current TW developments with Tsai not reaffirming 92 consensus.
TW isn't recognized as a sovereign country by anyone who can realistically intervene, so yes PRC can unilaterally and legally under UN framework decide to reunify with TW by restarting Chinese civil war that has never concluded via armstice/treaty - it won't be an invasion or annexation legally.
