There's only one dimension-traveling imp as far as I know, so it's an exaggeration to say "every imp" has access to another dimension.
There are plenty of glaring contradictions, so much so in Divinity 2 that I suspected that Larian didn't get anyone to actually play through Divine Divinity before tossing in references to it.
Zixzax-the-Almost-Wise is completely different, different appearance, different skin tone, different reason for his name, and he claims that the Divine stole HIS teleporter stones, when one was in a locked room in the Aleroth catacombs and the other was in Lanilor's closet. Zizzax was the one who stole them from the Divine - and returned them later without ever making a claim of ownership.
The Engineer in Divine Divinity didn't come across to me as a millenium-old supernatural robot, but apparently he was.
Thelyron Hashnitor's journals gave the impression that he was alive when he made a deal with Mardaneus and was only dead for a decade or so, but Behrlihn says he spent "a long undead life" in search of chaos magic.
There are others, but I'd have to look them up.
Each game adds and subtracts entire sentient species that were never referenced before (and/or are missing entirely after). The Divinity universe doesn't "play fast and loose" with the lore, each game seems to have different lore which is all piled up into a disorganized heap.
Well, heck, NOTHING looks or sounds the same between Divinity and Divinity 2. In that single respect I'd say your standards are way too high in what constitutes a glaring contradiction.
Now, about the imps...mine was an either/or statement. Not many are actively traveling, true. Zixzax is. Zaknadrix and Antx have/are in the magic sphere that gives entry to the Arakand universe. The imps that assault you just after becoming the Divine One disappear by doing a backflip into thin air.
I'm not sure they have a homeland because everywhere they are, they seem to be there as intruders or exiles. There's the imp village on Nemesis. The ancient castle in the Dark Forest. The hordes in the Wastelands. And then there's the really old PR material from before Divinity's release:
"The little devils have mysterious origins, with nobody knowing exactly where they came from. Theories range from the Imps existing as magical beings, to being experiments gone horribly wrong, to being former inhabitants of the fairy lands, expelled for their obnoxious behavior. None of these theories is really verifiable, however, since Imps have a particularly unpleasant disposition."
The rest I won't comment on, because, apparently, it's the subject of other threads.
