The adapter ring has the usual thread and can be easily screwed onto extension rings. However, vignetting can occur with wide-angle lenses. 

Yes. 

This particular model of adapter allows you to use lenses which do not have a dedicated aperture ring. These adapters do not have any electronic connection to the camera and hence cannot "tell" the camera what aperture is set, therefore the adjustable ring on the adapter provides a means of manually adjusting the amount of light passing through the lens to the camera sensor. NB If you have lenses WITH an aperture ring you could use another simpler (and cheaper} model of adapter and manually adjust the aperture on the lens. 

Try Conkin that are showing one at 55mm 

It can't not be fixed well with the lens without the adapter ring. you should buy another adapter ring of 95mm size if you want to make this square filter fixed on the lens.