Can nicknames be changed? I just put in a usual nickname (Moilleadóir), but it doesn‘t look like your system is capable of displaying (storing?) ISO-8859-1 characters.

I changed your username. FWIW: ISO encodings are long outdated, please make sure to only use UTF encoding.

I also wondered about this because at one point I had 4 entries but could not add any of my own, during the registration process.

But somehow, I do not know how, lateron, I ended up suddenly having a way to pick a user name on the forum. It was very confusing.

It may be better to either not ask people at all about it; or if it is done, to present a full overview where they can customize stuff.
Or perhaps even better, just tell them that they can go to "preferences" when they want to change something. Another option
may be to extend the drop-down checkbox to also allow custom entries. If this is too hard for HTML, a new fields below the
dropbox could be added aka "Here you can type in the nick you wish to use".