(1) The rear sight distance is changed by simply sliding the aperture up and down the ramp with your fingers.
(2) The lever on the left side of the receiver is a magazine switch, commonly called a "cutoff". In the "ON" position, the magazine is switched "on" and the rifle will load cartridges from the magazine. In the "OFF" position, the magazine is switched "off" and the rifle can be loaded as a single-loader. With the cutoff (switch) in the center position, the bolt can be removed.
(3) Yes. The varnish is a very common VFW finish.
Hope this helps.
J.B.