Jadra, the example you describe -- a profile with better info than the locked duplicate -- is something we need to hit hard with the other curators.
A simple solution would be to unlock the profile before merging. Then, follow up with a message to the original curator, "I took over your MP because I had to unlock it temporarily. Please take it back now."
I think our curator etiquette lets us do that. I do it to other curators every now and then, and they do it to me. (Sometimes you really have to change an outdated note, or merge duplicate MPs, so why not when you have to merge a profile with good information?)