Are there any US military personnel in Taiwan?

Officially at this time, no. There are security personnel at AIT offices (equal of a consulate). They don't wear US marine corps uniforms.

Today you don't see anyone wearing US military uniforms walking on the streets in Taiwan.

From 195x to 197x, there were US military consultants, and air force stationed in Taiwan.

In 2011, immediately after Japan earthquake/tsunami and nuclear power plant trouble happened, some US military personnel were vacated from Japan and stopped by Taiwan for a short time of few days.

During Taiwan 88 flood in 2009, US military heavy helicopters went to Taiwan help moving heavy equipments into the landslide mountain area.

Yes. There are active duty US military personnel who visit Taiwan and those who are "retired" or "on leave" who work there. I have met several.

Of course not. Had America sought such dangerous action, would at once launch missiles to wipe out both traitors and enemies in one fell swoop.