for #2, it is common to use a bar-end type control mounted in a handy place for the stoker to apply the drum brake as a "drag brake" for long descents. That way you can have separate controls for the rim brakes, like on a single.

for #3, and staying with freewheels, the Suntour Winner and/or New Winner models may be the way to go. You may need to go with a narrower, say "8-speed", chain for the rear. As long as the height of the new freewheel is same as the 5-speed, it will fit. Going to a really small (you probably won't be able to find less than 12, and maybe not less than 13, teeth for the smallest freewheel cog), there shouldn't be a problem of chain dragging on the chainstay. Might need to move a spacer from one side of the rear axle to the other, and potentially re-tension to keep the rim centered on the hub. With the Winners, and if you can find a few freewheels with different cog counts for donors, you can pick and choose cogs a bit more than the "pre-manufactured" standard offerings for many cassettes.