hi,we do not have filters for spotting scope. 

10x50 Powerful Binoculars for Adults with Low Light Night Vision, Large Eyepiece, 10 Seconds Quick Focus, Waterproof Wide Angle Compact-Binoculars-for-Adults-Bird-Watching, Hunting, Concerts 

That may be caused by the fact that the lens weight is much bigger than camera, If used your telephoto lens with our adapter ring, we suggest to use with a lens bracket and better to shoot on a tripod to increase its stability. 

The longer Flange distance lens to the shorter Flange distance camera, then could use the infinity focus.
but if the shorter Flange distance lens to longer Flange distance camera, then could not use the infinity focus, you can focus on a limited distance.
the position of the infinity mark 【∞】of the telephoto lens or super telephoto lens is shifted slightly to the positive side to enable focus adjustment even when focus shift is caused due to temperature changes.The shift range is denoted by an L-shaped line, Be sure to check the focus through the viewfinder even when you shoot at infinity. 

Yes, just make sure if you use a large lens use the lens' tripod mount. If not it may topple. But the tripod is sturdy.