You can in theory, but I think you’re unlikely to in practice. The OP invites us to speculate what a multi-party US might be like, and one of the things that might characterise it, fairly obviously, is that it’s quite likely to have a different electoral system.
So? Single party governments are often elected on the strenght of bad manifestos. My claim was not that multi-party systems would necessarily make for better government; it was that they would make for more consensus-seeking, and less polarised, politics. And if someone is of the view that electing Mr Trump is bad for the republic, they might well think that more consensus-seeking and less polarised politics would be good for the republic.
It’s important to Ireland, which is why the measure of the Irish political system and culture is how well it serves Ireland. But I didn’t mention Ireland because I regard it as a norm for the world, but because I’m familiar with it, and it’s one counter-example to the claim you advanced. It isn’t the only counter-example that I mentioned.
Well, I could pick nits and point out that poor old coalition-riddled Italy’s long-term economic growth and social development has outpaced that of the strong, sturdy, stable UK. But let that pass; mentioning Italy would back you up if your claim was that multi-party systems can produce political chaos. But that was not your claim; your claim was that “the only multi-party government that works is GB”, and that’s nonsense. Firstly, because examples of countries with well-functioning multi-party governments abound. And, secondly, because the UK mostly doesn’t have multi-party government. In fact, right now, it has a single-party government, and if you were trying to make the case that single-party government makes for stable politics and sound government, the current UK government is emphatically not the first example you would pick. It’s a basket-case.
You could make that argument to oppose any change at all. In fact, conservatives characteristically do. But if you take the view than anything at all in the current US political system is suboptimal, and if you want to make it optimal, then you have to be ready to make some change. The OP identifies a problem - the election of Trump - and posits a switch to a multi-party political system as something that might tend to avoid that problem. I think that, yes, it might tend to avoid that problem, because a multi-party system tends to incentivise and reward consensus-seeking and coalition-building, and a politician like Trump will not thrive in a system which tends to do that, since that’s pretty much the opposite of what he does. I’m not claiming that it would solve all problems, or make everything universally better; just that it might be beneficial in addressing this one.