A:AnswerNo. The M50 is an EF-M mount, this is an RF mount made for full frame R series bodies and some cinema cameras. You could potentially find an EF-M to RF adapter, but your best, and cheapest bet would be finding a compatible lens that’s already EF-M. Search “EF-M Lenses” to see your options.
A:AnswerThis is made for Canon's cameras with R in the name. For the M50 you would want an M mount lens, not RF
There is a possibility you could find an adapter to make it work however.
A:AnswerThat is normal, if you don't want your lens to hunt for focal point. You need to got AF manual set up page 4, and turn the Lens drive when AF impossible to OFF.
A:AnswerNo, there is no way to adapt RF lenses to the EF lens mount system. You can adapt any EF lens (even EF-s) to the RF system, but can't go back the other way.
A:AnswerIf you already own the EF f/1.4 and adapter, then I wouldn’t upgrade until the adapter or lens breaks.
As far as the EF-S 40mm f/2.8, that is a crop sensor lens while the RF 50mm is full sensor lens. With the 1.5 crop factor it will be like a 60mm lens on full frame camera. So the shot will be slightly tighter than the RF 50. The RF 50 will have an extra stop of light available to it so it should perform slightly better in low light situations compared to the f/2.8.