I’ll group your points together and answer for succinctness (hopefully some here will pay attention to this concept):
Currently, how many Militarily Useful Commercial Ships (MUCS) operating under US flag were built/are being built in the US? Ans: Not many as there is not the proper incentive structure in place to make that happen. Pure supply/demand economics should not be applied here as this is a national security issue (as noted by 60 min and the US Congress {i.e., the SHIP’s Act}).
Why not have allied nations build our MUCS for us? Ans: As the 60 Minutes piece succinctly stated, this doesn’t solve the problem. For the US, we need to be able to produce our own MUCS on our own soil. As the recent pearl clutching from NATO has shown, our “allies” can’t always be depended upon. Specifically in this case, S. Korea’s and Japan’s geographic proximity to China makes them particularly unreliable as producers of ships during the inevitable conflict with China.
“Most JA tonnage are not MUCS” Ans: You are correct. Which is why I found it curious that you would later post a graphic showing how the US is the 4th largest ship owner in the world by ship value. If you’ll analyze you own graphic just a bit, you’ll see that ~74% of that value is derived from cruise ships, OSVs, OCVs, and MODUs; N/A to this conversation.
US shipyards have foreign ownership. Ans: Doesn’t matter. In any type of crisis, they can’t box up the yard and go home when it’s on US soil. Once again, the key being the yard being in the US.
Can the US crew a bunch of ships? Ans: Show me the money. Sure there are other factors, but #1 is money. The relative value of seafarer pay in the US has eroded significantly in the past 40 years and everybody can’t seem to figure out why there’s a shortage of mariners. (???)
Would a fully US built/owned/crewed ship be competitive or would subsidy be required. Ans: No it wouldn’t as first world standards bring first world costs. Once again, that question is applying pure economics to an issue that has non-economic goals and factors.
How about foreign crews? Ans: No, not because of lack of ability, but because of potential lack of availability in a crisis. Once again, we need to be independent on this. Make the career attractive, and people will come.
How much cost is the US taxpayer willing to pay when foreigners will do it cheaper? Ans: To be determined. All we can do is make the case of why it’s important and the process will proceed from there.
In general, it seems you’re trying to apply concepts employed by other nations to the US. Other nations don’t have the military we do and all the logistical needs that come with it. The closest comparison is China and you can bet they’re not in any hurry to depend on another nation for something as important as shipbuilding.