@ainali
I've already opened a task and I'm following the rocky road, thanks. The unification of accounts worked like a charm, except commons.wikimedia.org. I ended up waiting for IDM account creation, and as it hadn't worked the two times I tried, I went to bed; I'll check again if it's already created soon, if not I'll update in the task.
I wasn't aware of the account migration, it's ok, a complex distributed plan like this can have some thorns. It'll get fixed.