The Taiwanese government can certainly allow Chinese soldiers to do whatever they want within the borders of Taiwan.
There are no de jure borders of Taiwan. Both the PRC (which currently governs mainland China) and the RoC (which currently governs Taiwan) (officially) claim that both mainland China and Taiwan are part of one nation and each (officially) claims to be the sole legitimate government of the whole nation. Also, the RoC used to control mainland China and was expelled from it in a way (IIUC) not completely unlike the former Ukrainian government or Obama in your hypothetical.
As far as my hypothetical goes, I think Taiwan has the right to ask the US to have US military stationed within Taiwanese borders and that the US can accept such a request from Taiwan.
I don’t think that would violate Chinese self-determination in a meaningful extend.
There are no de jure borders of Taiwan. Both the PRC (which currently governs mainland China) and the RoC (which currently governs Taiwan) (officially) claim that both mainland China and Taiwan are part of one nation and each (officially) claims to be the sole legitimate government of the whole nation. Also, the RoC used to control mainland China and was expelled from it in a way (IIUC) not completely unlike the former Ukrainian government or Obama in your hypothetical.
As far as my hypothetical goes, I think Taiwan has the right to ask the US to have US military stationed within Taiwanese borders and that the US can accept such a request from Taiwan.
I don’t think that would violate Chinese self-determination in a meaningful extend.