I have seen the rather grim fan theory that there is no such thing as a Muggle-born - rather all children believed to be Muggle-born are the result of conception under the influence of love potions, followed by the application of Memory charms.
That’s what makes my suggestion a fanwank: if we actually had canon authority for the full context and ramifications of love-potion use in the Potterverse, we wouldn’t need to fall back on speculations.
Nope. Sometimes, but by no means “almost always”, at least in the books. It’s commonplace for almost any teacher to call a male student by surname only, but it’s literally unheard-of for any female student to be so addressed. The books are full of examples like the following: