Those videos are very entertaining. In my 37 year career I have worked a lot of snowplows in the southern Ontario area for CPR.
To reply to your questions (at least the way CPR operates them):
1/ The power for these trains is decided by the following criteria: what is available, any restrictions on the routes to be plowed(e.g. some lines can only handle 4 axle power) the amount of snow (some locations may require 2 or 3 - 4 axles to get through deep snow) the need to reverse at the end of the line ( some lines end without a wye or turntable). without turning facility at end of line, one engine must be able to operate both directions (with a pilot and headlight at both ends) or two or more units must be used with outermost units facing outward in direction of travel (one each way).
2/No special license is required. The plow crew are track maintenance staff, that normally maintain and repair track, qualified for that region. There is a plow foreman (needs an A rules card same as a Conductor) and a helper, ( needs a B rules card, same as brakeman) .The train crew is same as any other, minimum one engineer and one conductor, maybe a brakeman too, depends on contractual situations versus actual needs. The crew is local to the area, qualified to operate any trains on that trackage. Experience on snowplows is helpful because there are some different needs, for example brakes need to be conditioned (used periodically to keep shoes warm, to ensure effectiveness, and prevent ice build up) The signs you might notice along rail lines in Canada with 2 dots on a rectangular background, are specifically for snow clearing operations. They advise of locations where the wings and/or nose of the plow need to be retracted. Typically, these are at crossings (private and public) and bridges.
I found these jobs enjoyable, because they represented a change from the routing. Some guys hated them. I found them to be fun and well paying. I hope this info is useful.
I've gone into snowbanks in run 8, But speed is more relevant, versus throttle position, but usually it was run 8 to attain the speed needed. The plow foreman used to call on the radio for "more speed", or "give all you got, she's heavy". etc. Even on branchlines normally restricted to 35 mph, I have hit the drifts at 55 or more, and stopped partway through. Sometimes a few runs were needed to get through.