(that's how you call it in english?)
There may or may not be an actual name for it, but I don't know what it is. Wikipedia's article on Ö doesn't have any formal name for it either, and just refers to it as "the letter O with umlaut," so that's probably just what it's called
Anyway...
1. I probably could get used to the strange graphics that just turning off the tileset causes eventually, but I just don't want to
For whatever reason, it really bothers me when things just don't look like they "should," especially when you've got really strange stuff showing up for everything. In vanilla, there's a fairly limited set of tiles used for stones, for instance, and tilesets add many that really don't make sense.
2. Alright, so it's a more involved matter than I had hoped in other words
I might try it out when I gen a new fort again (which, depending on how successful the last stages of my current megaproject are and how much I play this weekend, might be soon) and try just using your modded creature files. I can't tell from a cursory glance through the raws if any assorted critters' graphics were actually altered instead of just overwritten by the graphics, but if not, that'll probably satisfy my needs well.
I'll probably end up using just the body plan and creature raws, since those are, as I see them, the most "original" parts of your work here that don't cause the annoying graphics issue, but I have high hopes for those parts at least