it fits in a 100 mm square filter mount, so it depends on the size of the mount adapter to the lens 

sure. it could mount a 1/4 for gopro. 

no it will not the eos-r has a different mount 

The V50x comes with a skeleton case with a slotted rear door replacement, but there is no opening to allow plugging in an external mic. As a note, the sound recording, even while in the completely waterproof case, is better than I had expected. If you're not worried about moisture, there is a threaded tripod mount on the bottom of the camera and an adapter that will allow you mount the camera without using the case. 

Yes