Probably yes for both questions. Ch.4 of Hacker's Delight starts with assuming all quantities are signed integers. A little further along, a lemma about computing b-a when a≤b is introduced:

Lemma. If a and b are signed integers and a≤b, then the computed value b-a correctly represents the arithmetic value b-a, if the computed value is interpreted as unsigned

It's been a while since I worked through any of Hacker's Delight, but I think this means that unsigned values can be used to check bounds of signed and unsigned values. This post is an improvement over that work, so this same property should hold for these new bit-twiddling equations. Still, I'd run a few test to make sure.