> and the situation under a supposed "merger" (not that it will happen) can still get much worse

Well, hypothetically speaking (given we both acknowledge a US-Mexico merger is rather unlikely to ever actually happen): yes, it could be worse, maybe even “much worse”; but it could also be largely the same; it could even be better.

One way in which it could be largely the same: the remaining racism in the US is largely of the subtle/systemic kind rather than the blatant apartheid/Jim Crow kind, which makes the racism problems of the US and Latin America much more alike today, despite historically the former being drastically worse than the latter

One way it could be better: for all their limitations, American tribal governments actually have significantly more powers than their Mexican equivalents - federally-recognised US tribal governments are partially exempt from state jurisdiction, while all Mexico has is indigenous-controlled local governments with significantly less legal autonomy. So you could argue that the US shows greater (albeit still limited) respect for indigenous peoples’ rights to autonomy and self-governance, and if a merger of the US and Mexico happened, it might result in that US approach being adopted in Mexico too, resulting in improved rights of autonomy and self-governance for Mexico’s indigenous peoples