1) When multiple overlapping or adjacent regions are highlighted the result should just be a single highlight over the union. This is how highlighting works in e.g. Kindle and Apple Books. A common workflow is to highlight an area, and then realize you made a mistake and wanted to highlight adjacent words. Right now this gives two overlapping highlighted regions with a darker shade on the overlap -- which is distracting, and it's highly unlikely that the user cared to generate two overlapping regions (and pretty unlikely the the user cared to create two adjacent but distinct regions).
2) I'd like to be able to easily select text which is a subset of a highlighted area. This actually impossible right now because the highlighted region is treated as a single selectable object, so if I click into it I can't select a portion of the text. Also if I wanted to copy out a span that is partially highlighted, I have to remember to start the selection outside the highlighted region, because clicking on the highlighted region doesn't put my into a text-selection state.
Overall you should simplify your highlighting UI -- just treat it as a flat annotation. Yes I need to be able to delete highlights, but I don't want them to become top-level entities that prevent normal text interaction and do not merge with each other (in the natural way that actual highlights in a printed book simply aggregate).