I don’t understand question 2 of your extra questions so you might wish to expand on that, but for question 1 it’s not easy to export highlights from Apple Books so you might want to research that first as there are some workarounds and all are differing levels of annoying. Apple Books doesn’t play nicely. (My personal view is if you’re serious about reading and removing your highlights, avoid Apple Books because messing about with their thoughtless architecture will get old real quick. Obviously that doesn’t help much for highlights you already have trapped in their ecosystem. If you’re only talking about occasionally needing to remove notes, you may not mind.)
For your title question, importing a html file of Kindle Highlights, it’s no different to importing any other file type into DT, but I’d first ask if you actually want to store your Kindle highlights as an html file. I dont store anything as html myself, but this is very much a personal choice and many people do. If it was me, I’d want to store my Kindle notes in the same format as the other note files I work with (my notes are mostly in markdown with some rtf).