Diameter of the tube leg is from 19mm to 28mm. 

Yes. I have the same lens and adaptor on my Sony a7ll and it works beautifully. The m42 is also known as the 'universal' mount as other 3rd party camera and lenses also used it. I own 6 other K&F e-mount adaptors to different lens makers (ltm, MD, CF/CD, Nikon) and they all are well built with a nice snug fit. Highly recommended. 

67mm should also be stenciled on the front edge of the lens somewhere. If so, then you should be good to go. It should be on the lens because a lens cap can be lost or broken. 

I actually have not received this adapter yet. It's shipping from China. Don't think that was disclosed in the ad. However, I do not believe this is for the Pentax Q body rather for digital and film cameras that take the Pentax K mount in its many variations. So I'm expecting to be able to mount my Leica M mount lenses on my digital and film Pentax K mount cameras, no autofocus or relay of exposure information, only to be used with manual focus and exposure. 

Yes the FD-NEX adapter is what you need. I also have a a6000 and a Canon FD 50mm , 28mm, and a 300mm and they all work well with this adapter.