Adding indoor data for all TUM buildings sounds like plan. And using the data in NavigaTUM it's a cool usecase of OSM data. 
I also thought about adding more indoor data at TUM, but as there are not that many Maps that support indoor data and TUM students rather use NavigaTUM I didn't do it and prioritized underground stations (e.g. Sendlinger Tor or Müncher Freiheit) instead.

About the editing tools: For more advanced indoor mapping there is no way around JOSM. In comparison to the ID editor JOSM can filter by level (without level filtering you can't handle places with several indoor floors on top of each other). About OsmInEdit I can't say that much, as I only tried it out just now.

And thanks for the clean up of the StudiTUM, the indoor data is much better now!