awl51

I also would think about a mixed solution.

Sounds to me like there are 3 critical constraints for any equipment to contend with in this particular situation: grade, turning radius, and where to relocate the snow.
You'll want to be doing productive work on every pass, and have something that has traction and is still doing work as you do the "up" pass on each drive. Because the areas are narrow as well as steep, a tight turning radius will minimize unproductive time spent jockeying the equipment around.
And for snow relocation....well, it sounds like a blower has to be involved there somewhere. Given the building density, just pushing the snow around appears not possible; however, a plow is still typically  far faster than a blower.

The previous contractor's setup sounds very effective, but those things seem a bit spendy.  If startup $$$ are a little tight, you might want to consider keeping the ATV rig and use it in combo with a robust blower.

One strategy: Use the ATV plow to make a couple quick passes on each drive to windrow the  snow; not to 'pile' it, but to windrow the snow cover to  your blower's maximum depth - what you want at the end is a set of large clear plowed areas combined with a small set of snow rows that are just below your blower capacity. Then blow the suckers.
In a light /medium snow, you will bless your ATV, because you may end up with only 1 or 2 windows per drive to blow (as compared to going over the whole thing).

I'd for sure get a remote angling kit for the ATV plow if you don't already have one. And if you want really happy customers whose steep, slippery drives are squeegee-clean after each storm, check out a down force kit for the plow as well.

Good luck.