Apart from the topic, it is actually not really clear what percentage of the global hashing power is controlled by chinese miners, and which of those could possibly be coerced by the chinese government to refuse to mine such transactions (if they can reliably be identified as originating from the U.S.A. at all). Pools might not get away with such a change - as a pool miner, you can switch pools relatively quickly if you don't like their behavior, making the pool irrelevant in a short time. Operators of actual mining hardware within China would be more susceptible to such a coercion, but even though they control quite some mining power, they would most likely only cause some delays in transaction processing
I think it is safe to assume that they have enough hashing power to affect the majority of transactions if they choose so. This is not really a question of whether they control over 51% of Bitcoin hashing power (personally, I think they control substantially more than that), since even 30% could potentially make unbearable the lives of those whose transactions get rejected or postponed. Regarding the coercion by the Chinese government, I don't really think they will have to coerce anyone. As stated in the OP, rejecting certain transactions will be a retaliation of sorts, so the government may have only not to object to miners retaliating on their own in case Trump does really decide to start a tariff war. That's why the issues raised in the opening post might have substance after all. I won't forget how in July, 2015, thousands of unconfirmed transactions got stuck in queue (metaphorically speaking) with lots of new blocks containing only one transaction. Some euphemistically called it "testing Bitcoin limits" later...
Now try to tell me that there is no collusion between miners
So unless we have hard evidence that a significant percentage of miners implement policies that discriminate against some group of bitcoin users, I would file this concern under "What If?". There are probably a dozen "What If" posts each week on BCT, and it just does not make sense to act on them. One should probably look at the more realistic scenarios and think about possible workarounds, but worrying about the rather unlikely cases won't help much
I've worked with a few Chinese businessmen as an interpreter, and I have no illusion that they will retaliate, with or without coercion