I would say yes, that would fit:
- My MTB has 117cm Radstand and fits easily.
- Veloträger is 118cm wide (Felgenhalter Ende zu Felgenhalter Ende) but since both are under an angle, the supporting position (Kontaktfläche Felgenhalter - Reifen) for any MTB is far less than the Radstand.
- At some point the Schnallen could be the limiting factor since these would be too short. Mine still has a lot of spare though (2.4 Reifen) and therefore I do not see this as an issue for a 120cm radstand MTB.

Yes, no problem at all! The width can be adjusted.