Re: What is the risk?
Not really, once they've bought the debt then the US doesn't really care what they do with it. Anyway, for some years now China has been diversifying into non-Treasury assets. But, much as it was with the Arabs in the 70s and Japan in the 80s, America doesn't really care who buys the assets just as long as they keep paying in dollars. Majority ownership of strategic assets is routinely ruled out.
The real risk to the US is a shift away from using the dollar for settling accounts. While China has made some small progress in getting deals in raw materials denominated in other currencies or forms, the vast majority of all international trades are still settled in dollars and this gives the US enormous power.
If the spat with China wasn't accompanied by other spats with America's allies then it might be easier to get the allies on board with a coordinated approach against Chinese more dubious business practices. As things stand, however, the US is doing a great job of losing friends and influence. If they continue at some point there will probably be several special purpose vehicles (SPV) for settling some trade that the US disapproves of without using dollars. The bullying over Iran does seem to enjoying success in Europe with many companies voluntarily withdrawing from deals with Iran, but other countries, notably India, might be less impressed. A pipeline from Iran to India and possibly even to China is probably sill a pipedream but people are no doubt working on potential workarounds such as trading Iranian oil via Iraq.
I'd love to think that a war with Iran was completely off the table (it is completely unwinnable for the US) but the combination of Trump (fuckwit) and Bolton (nutjob) mean that almost anything is possible. After all, what could be better for US shale than a conflict in the Gulf?