1. The US can't defend Taiwan from China without attacking the Chinese mainland. Which leads to a collapse of the global economy and nuclear war.
Nonsense. USA can provide weapons to Taiwan which would help to defend it greatly. Especially if USA would do it now before the hypothetical Chinese takeover of Taiwan starts and before arm deliveries are exposed to Chinese attacks. If enough weapons are provided then the Chinese attack is not even likely to happen since it would fail against a strong defense.
Providing weapons to Taiwan does not even require USA to target anything Chinese ... not even mentioning the mainland. If China attacks then collapse of global economy is very likely. Nuclear war is not a given consequence at all.
2. Taiwan will join China sooner or later because their future is with China, not the US. Xi is well aware of this, which is why he's in no hurry to invade.
Are you reasoning based on the unproven claim that "Taiwan's future is with China"? You need first to show that the claim has any merit. Xi is claiming this because it is good for his (especially domestic) politics. He is not attacking Taiwan because his knowledge of the future is given which allows him not to hurry. He is not attacking because he does not have enough landing ships yet. Bad Taiwan attack progress would also destroy his economy and domestic standing more than the domestic morale boost from the unification. If Xi would be ready to execute easy and swift Taiwan takeover now then he would just do it now.