I dunno the answer, but can you use the knife cut, using midpoint, then cut through the lines you want, and if you need to adjust the verts, scale them in the right axis?
And yes, that may very well be the worst run-on sentence I have ever written in my life.
knife cutting using midpoint allows me to place an edge loop in the middle. But I havent figured out a elegant method to move the edges outwards. Scaling in x-z direction with the cursor set to the center, will scale the round part okay, but the base of the arch is also scaled downwards.
I tried to re-create your problem by making a similar arch in 2 and 3 dimensions, and can loopcut each with Ctrl-R no problem. Could you post a .blend so we can see what’s different about your arch?
i don´t know why, but I tried to reproduce it (since I didn´t save the original file) and now it seems to work perfectly!? Thanks anyway!