Why is Taiwan specifically a threat? They're a long-term ally and their voters recently gave a new mandate to anti-China leaders in an election. If the US is worried about threats from Chinese electronics companies like Huawei it doesn't make any sense to alienate Taiwan.

Taiwan isn't the threat. China annexing (or otherwise disrupting) Taiwan is.

At which point the US can nationalize the fab on US soil. That makes a compelling argument for forcing fab creation here.

Not if you're Taiwanese.

Chinese espionage in Taiwan, perhaps.

Then again, maybe that's a good thing; certainly would encourage greater counterintelligence cooperation in Taiwan.

In addition to the other points, it seems like it would be much easier for the Chinese to infiltrate Taiwanese industry with spies/saboteurs since they share cultures.

Blackmail of key individuals is probably easier too since Taiwanese are more likely to have family members on the mainland and/or Hong Kong.

Not so much specifically as in politics alliances change fast so you have to consider even your friends as a potential threat. After WWII it took only a few years for Japan to go to friend - there was no way to predict that (as opposed to isolationists who want nothing to do with us)

Why?

So that there is a “chain of custody “ from plan to production completely within the US. Presumably the people working in such would be reachable by US law, if not closely aligned with their own country’s interests.

If the fab is in the US, we can take over the plant and use the existing workers who as Americans are loyal to the US over the company. Of course we have to kick out foreign company leadership, but there are plenty of leaders who can run a plant with the same processes for a few years while learning what is really needed. Odds are there will be some local leaders who can step up. The plant might fall behind the latest processes but it will be good for what it does for a while - long enough to finish the war