An amendment process that in practice is impossible to exercise is just as good as having no amendment process at all.
No, and no one is saying they can. The law says American companies can't do business with a certain foreign-owned company.
It is beyond settled in law that this is something that the US government can do.
Sure, the government can do that, and when doing so infringes on Americans’ speech or access to information, it introduces First Amendment questions that must be addressed.
“The government says CNN can’t post stories from BBC” isn’t immediately resolved by “it’s a foreign company.”
But this doesn't do that. Everything that Americans could post or watch on TikTok, they're still allowed to post or watch anywhere else.
Is the government allowed to shut down Harvard because the same classes can be taken at ASU?
No it isn't. The argument here is that it isn't a restriction on speech at all.