After several failed attempts to implement a multi-item-editor, it finally seems to be working. Needs still some testing, but the first results look promising.
The way it works is, that I will manually enable properties throughout the playfield for multi-editing. Those enabled-for-editing properties will then be visible and editable, as soon as you select multiple items in a list and click edit. If you just select a single item, the ‘old’ editor with all properties shows.
I think this is a manageable solution.