On nixon: might well be. But it's never happened so far, and that's because congress is hyper deadlocked by design
and a ridiculously high bar of non-proportionally-represented senators need to agree to convict. Just compare that to other democracies that can switch their executive with a simple majority in one house, yet remain stable.
As to congresses deadlock being a feature: it may well have been intentional; but it's pretty plausibly a long term fatal blow. Countries need to be able to evolve, and when small minorities can prevent that (and we're not their yet, but it's moving that way), you're creating really perverse incentives for horse trading, leading to really low quality law making, and thus low respect for congress. It's not out of the question that this might lead to the nation's dissolution in our lifetimes - what happens when elections are legally stolen?
On impeachment: we'll see what happens with trump who happens to be a perfect case to check whether this feature of the constitution is more than hot air; but given how stacked the deck is against conviction, I'm not holding my breath - and if it's not going to happen now, it never will - not only was he utterly shameless in undermining the election (knowing full well that democracy is worth fighting for, i.e. raising tensions dangerously) and addition literally using combative language and calling for as close to a stormin of the capitol as possible while trying to retain plausible deniability, we're also just post election that shifted the balance of the senate against him. This is pretty much the best case scenario for an impeachment, and it's far from a sure thing.
Additionally, the current election was upheld largely because of states still sticking to extra-legal solid traditions. Those however, are not constitutionally protected; i.e. what worked was american culture & tradition (sometimes backed by state law), not really the american constitution. I mean, for scale: it's easier to throw out an election than a president, which is absolutely Not Ok.
Some of your other objections relate to tradition, e.g. such as puerto rico not being a state. This has nothing to do with the constitution; this is purely a political tradition. And indeed; many traditions are fine - while I think there's no question on the specific instance of puerto rico deserving statehood, the point is that the constitutional bar is very low despite the fact that it allows senate packing. A malicious populist could easily abuse that, if convenient, were it not for the real protection: tradition&culture, not the constitution.
The US has a solid set of traditions and an extremely civic culture; and that's why it works - not because of the constitution.