For any1 who still want to use the right alt key, there is a solution for this.
You can edit the key config manually.
Open and edit the Osu!.[Your Name].cfg file in your osu folder.
Just look for following Lines: ManiaLayouts4K = [] for your 4k setup (etc.) and ManiaLayouts16kSplit = [] for double playfield setups.
This is what these lines look like:
Make sure that your Osu is closed when u edit it, the config file only gets loaded once, when u open osu and it will overwrite your current setting when closing Osu.
EDIT:
If u play with LConl and right Alt in your key setup, then you will always trigger both keys whenever u press right Alt, but you only trigger the LCon key, if u click that one. (This is kinda problematic for my 18k setup).