I went by again. It is as I thought, the entrance at number 24 is only for the community centre. The rest of the units in the building are accessible via number 1 on rue de l'harmonie.

Hard to say, the ground floor is all De Harmonie but indeed there are apartments and other activities in the building. My feeling is that this main door is only for De Harmonie and I didn't see multiple post boxes there.

Yes indeed, apologies for the typo.

Thanks for the explanation.
Noted for the address tags, that is clear.
For the presets vs tags point: I will from now on make sure to check the information is not already in the tags. So just to make sure I understand: a preset can be empty but the tag it relates to could be completed. Is that correct?

Thank you, this is very helpful! I didn't really know what it was for, so your advice is very much appreciated.