Thank you for your answers guys, I'll try to answer.
1 - Ok, I understand the issue of triggers, I had it before and I understand. Anyway, there wasn't any trigger to be met.
2 - The event is in an already existing file, so it is included in the txt list you mentioned
3 - I took the leaders from UoB, which was annexed by me. Then I released puppet UK. I copied the leaders from UoB csv Excel file, changed the leaders to unique ID. So I assume that they are "dormant", because they are totally brand new.
I'm writing from my phone now, so I'll test it when I'll be home. Any other ideas, I'll check them out.
Update: I replaced all "wakeleader" with the empty command, and the event works. So, it can be assumed that the problem is within the commands section itself. I don't understand why, then, what should be new leaders with unique ID don't appear. Their former counterpart were part of the annexed nation (but each one with different ID). What am I missing?
Final update: I came across a thread about leader editing and I saw that, unfortunately, I have to start a new game should i wish to see the new leaders, as the csv file is read upon country creation.
Nevertheless, even with already-existing and sleeping leaders, created by the modders, with unique IDs, this does not work. May you be interested, in the eug savegame I do not see any of the guys I want to wake