On the growth/return of white supremacy in American politics, I'd say it has a bunch of causes. Firstly there's a lot of right wing disillusionment with the Republicans and the Individualists, the former of which are really a less statist version of an OTL Christian Democratic party by this point and the latter of whom are more committed to libertarian economics than social conservatism. Secondly, it's worth remembering that a major target of the upswing TTL is not just anti-black sentiment but anti-Catholicism and anti-immigration feeling, especially targeting the "ethnic whites" migrating from southern European countries ("Spain isn't sending us their best" etc). I'd also point out that the ingrained proportionality of TTL's federal electoral system means that there was always going to be an opening for this kind of fringe party once enough people worked out that they could do it.
On the position of the US military, they've basically not fought a conflict since the Phillippine Uprising and that definitely shows. A lot of the equipment is either out of date or vanity projects while the men and officers are inexperienced. Of course, the same can be said for the Mexican armies, which partially accounts for the enormous civilian and military death toll in the American theatre.
