Really pity you lifted up a decal, Fredrik!
I for me hate masking on top of decals, which also caused trouble on my (still not fixed) Dr.I.
To paint shadows on top of a decal I have used a "moving mask" with a Post-It.
The upper side looks great so far.
Decals and tape are a real challenge, I know. I have to look for the pink tape because that seems to work.
I have a pink one form TESA, which is even more tricky then the trusted Tamiya. Not at the beginning, but after a while is get more tacky. I'm very careful with this now!
Cheers,
Frank