Jks, it's exciting to see some more love for the Northern clans (but you're right to be warned about how exhausting snow projects can be).
I like all your tests so far, it would be great if those houses you've designed could also be built diagonally, gives you a lot more flexibility with making the buildings fit into the landscape.
Since I probably know the pain you can expect better than anyone else I'm fine to approve your application, but it would be helpful to get the thoughts of another mod to get some fresh perspective on your plans.
Here's some lessons I learnt from my time in the North.
I tried out a few different styles of snow vegetation but this one turned out the best I think. It's a layer of oak leaves beneath the top levels of snow layers. Can be tricky on steep slopes though as the leaves become can be come exposed on the sides so it works best on flat areas.
I also tried putting down layers of carrot/turnip crops and then using WE to replace the soil beneath them with muddy snow, but it's not the neatest option since you can't put any snow on top of the crops and they'll all self-destruct if touched once the soil beneath them is removed.
It's easy to go overboard with the gravel/dirt/mud mix for village paths. Eventually I realised the style that looks best was a narrow line of dirt/gravel with a layer of muddy snow between it and the regular snow to make a gradient.
The roads between villages/hamlets/holdfasts are a pain to make, since you have to clear all the existing trees in the way then smooth out all the snow layers running along side it. I used a script that made it a little easier, but I haven't discovered the perfect way to do it yet. You'll also need to make sure you add slabs every time the road goes up/down a hill. Best advise I could give would be to make the paths before you add any more forests.
It took me a long time to figure out the best way to add snow to mountains. It's tempting to just get the gunpowder brush and /b snow the entire side of the mountain, but that invariably just scatters snow layers around randomly and doesn't look good (the snow brush also has a habit of sneakily spreading the snow inside houses without you noticing).
If you're starting with the all snow hills like this:
get a Northern Terrain block brush (/br sphere 2029:3)
Then add a mask of snow and the angle of cliff you want to make, e.g. /mask [2029:3]&/[50d][90d] and start brushing over the snow.
The last thing is to remove all snow layers on top of the northern terrain. Use an air brush (/br sphere 0) with a mask of on top of northern terrain /mask [78]&>[2029:3]
I also made some schemsets for snowy terrain /schset WWDetailSnow and WWDetailSnow2
When placing tree schems on top of snow, you'll generally always find that the tree sits on top of the snow layer and looks like it's floating, I never figured out what combination of schbr settings and masks to make the schem actually replace the snow layer, but I made a script that automatically fixes it and adds snow/icicles to the tree. I've also got a script that can do a kind of ok job at adding smooth levels of snow gradient over large areas, If you setup liteloader I'll send you them.
The gunpowder /b snow brush is the fastest way to add semi smooth layers of snow, but you can also use it to lay pebbles down for streams etc if you use a pebble brush and mask to replace the snow layers.
Say you wanted to add some pebbles down for a river, you could cover the area with snow layers, then use a pebble brush with the ID pattern: /br sphere #id[2212] with a snow layer mask: /mask 78
(You can then reverse it to mix some snow layers into the pebbles like this)