In addition to other answers:
"One first name is enough" – this is the first bullet point in name change policy on Finnair site and you already comply from the beginning, so there is no need to worry. I understand that bullet point in way that even if name change needs to be done on air ticket, it is sufficient to keep only your first name and remove other given names during the change.
In my EU country (Slovakia), additional given names are "second class citizens" and are required only on security-related forms, but otherwise they are not requested, even not on tax forms, medical records etc. which are prescribed by law. Unlike the first given name, you can instantly add/change/remove additional given names in quick administrative process for about 3€ fee. Total limit is up to two such names.
Based on the above two paragraphs, it seems that also the Finnair views additional names practically and they are not really interested in them, unless you make serious spelling mistake there, which is a blocker. To minimize room for errors, they encourage their clients not to use them at all. These names do not necessarily appear on air tickets even if entered into their system. I would not worry about that.