Questions about Sony La-Ea4

The 16-50 f2.8 is an APS-C lens and makes little sense on the A7RII because you would only have 18MP because the whole sensor is not illuminated. The LA-EA4 would also be the wrong adapter because it's meant for old rod-drive lenses and turns the mirrorless A7RII into an SLT camera with mirror film. Better would be the LA-EA3 without foil. However, I advise against using APS-C lenses on full-frame cameras. Makes no sense.

Yes, without noticeable loss of image quality at least. The adapter has a semi-transparent mirror. So you lose some transmission (a third of an aperture or so) and there is extra glass involved. Just like the A-mount SLTs from Sony. In my experience, the autofocus works less well than on a real A-mount camera, but it's still passable.

The focus speed is primarily dependent on the lens used. I bought the adapter at the time to continue using my old or very special lenses. It works well for me with the SAL1118 and the SAL70300. You can save the Fr 287.- and invest in a new and good E-mount lens, e.g. the SONY 24-70mm F/4.0 (SEL-2470Z) [my standard lens] full frame and fits perfectly, the largest aperture 4.0 over the entire zoom range is quite good and is sufficient for me.