Yes they can be taken off, you can buy more frames too, to fit other 100x100 filters and gradient filters. You only need to pull apart one joint and the filter can be removed, just pinch each section and pull. 

I don't know about buying pre framed filters but I bought the separate frames to put my own filters into. 

Yes but you have to order them from k and f directly. 

hi, this pro square filter system is not compatible with ordinary square glass filters. a KF protective frame will be needed. we will have the frame and pro square filters soon. 

Works fine throughout the range on my 10-20 wide angle lens on a cropped sensor. Although I suspect that 10mm would be the absolute limit, any wider and I think vignetting would occur. I don't use the extreme 10mm end that often however I was expecting some vignetting but I seem to have been lucky and gotten away with it. This is a really nice set of filters. Hope this helps.