Welcome to the fascinating world of wild European hedgehogs! It sounds as if you may be right – that the hog collecting grass may have been building a birthing nest somewhere.
You are right that hogs are solitary but if we provide food, there may be congregations of hogs i.e. we sometimes hear about as many as 6, although some of those may have been youngsters. But 2 or 3 are not unusual. It’s impossible to say whether the ones you saw were male and female without more information, but they could have been – although could also be 2 females. (2 males would be more likely to be intolerant of each other, especially if there was a female around, and one might have been likely to biff the other).
Re. hog houses. It sometimes takes a while for hogs to accept them, but if you put a handful of material inside (for instance medium sized leaves) and leave more suitable nesting material in the vicinity, it sometimes gives them the idea. In general, it’s best to keep feeding in a separate area to boxes you would like to be used for nesting – because food may attract predators. A female hog with young may also not appreciate having boisterous males just outside her door!
Good luck and happy hog watching! Fingers crossed for some hoglets to appear a bit later on.