How do you keep glued snow from flaking off the trees?

The matte spray mist is exactly the right instinct, you're almost there. What I do on my trees is hit the branches with the white glue and flakes like you did, then once it's fully dry, come back with a second pass of diluted glue applied with a soft brush or a spray bottle set to the finest mist you can get. The second coat is what locks everything in. One pass of glue underneath only holds the flakes that touched the glue, so everything sitting on top of the first layer just waits to fall off. Diluted matte medium or white glue cut about half and half with water works fine. Your test branch proved it holds, and the dulling you noticed ussually disappears once it dries completley, matte medium dries pretty invisible on white flakes.

One thing that helps a lot before any of that: apply the flakes in thin layers and press them gently into the wet glue with your fingertip or a dry brush rather than just sprinkling and leaving them sitting loose. More contact means less shedding later. For the drifts at the tunnel portal, yes, wet water and a pipette works, that's the standard way to do it. The trick is patience. Wet the pile thoroughly first with the wet water so the glue can wick thru instead of beading up and sitting on top, then drip the diluted white glue on slowly and let it soak in on its own. If you rush it or flood it, you get a shiny glue puddle and the snow can discolor. Let it dry a full day before you even breathe near it. Deep piles might need a second session of pipette work to get glue all the way down to the base. If a drift still feels fragile after that, a very light mist of the same spray you tested will stiffen the surface without taking away the fluffy look. I've also had luck mixing a few drops of dish soap into my glue mix as a surfactant substitute for wet water, works about the same.

First post here so hope I'm doing this right. I don't run trains, I do a little Christmas village on a foam board each year, and I had the exact same shedding problem with the snow on my rooftops. What saved mine was just leaving it alone more than anything, honestly. I noticed mine shed worst right after I finished because I kept nudging things to check them. Once I let it sit a full week untouched it seemed to settle in and stop dropping. So maybe part of the fix is patience too, not just more glue. Do you guys cover your layouts between sessions or is that asking for trouble with moisture?

Tried the pressing-into-the-glue thing on one of my spare trees last night instead of just sprinkling, and wow, way less fell off when I picked it up. My fingers got pretty sticky though, half the flakes ended up on me instead of the branch. Is there a way to keep the glue off your hands or do people just wash up a dozen times? Also curious about that dish soap drop trick, does it matter what kind of soap, like would the fancy lavender stuff mess anything up or is it all the same once it dries?

On the drifts, one thing I started doing after a couple of ruined piles was building the shape over a crumpled foil or paper towel form first, so the loose snow is only a shallow shell rather than a deep pile the glue has to travel through. The soak gets down to the base much more reliably that way, and the underlying form does the structural work. As for the sticky fingers question, I keep a small bowl of water and an old toothbrush at the bench and rinse as I go, gloves always felt clumsier to me than just washing up. And any plain dish soap works for the surfactant trick, just avoid the kind with added moisturizers or whatever, those can leave a faint sheen on the flakes.