I question if #2 is some indisputable political *truth* or rather something we all just assume will always happen because it did once in such an important and dramatic (and drawn out) event: Southern Whites abandoning the Democratic Party.
It happened in West Virginia and Kentucky as well, although they are technically in "the South," they aren't usually thought of when it comes to what you said
(or are they?). There are also a ton of Romney-Clinton legislative seats in New York and Pennsylvania collectively where Republican incumbents held on fine in 2016 but may be endangered this year and into the future, so we'll see what happens there. In fact, there are seats like this all over the country. When the coalitions shift, it can take a couple election cycles or more for the party the districts shifted towards to actually win them downballot. What made the South unique was just how long these legislative/House seats stayed with Democrats. It took Republicans generations to break that hold.
Also I think Virginia (2008-2017) counts differently than the old Southern Democrats dealio. This time it was Republicans who held onto seats they were doomed to lose eventually, and part of the reason it took Democrats until post-Obama to win them was that in some cases they didn't even challenge Republican incumbents in off-year elections like 2015, but also because the Democratic base was demoralized in those cycles and some of the voters they were winning at the presidential level were still supporting local Republican incumbents, maybe even just out of habit.
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At any rate, I don't know if this happens with every demographic / everywhere, but it does happen enough to believe in it. I also don't think every single presidential win means as much as the numbers say in one election. For instance, Obama won Michigan by a huge margin in 2008, but many of the legislative seats he won quickly snapped back to supporting Republicans as they were never going to stop doing that in the first place. They only supported Obama and maybe a Democrat or two temporarily because of a short-lived backlash against Republicans.
