Will real feathers on the trailing edge mess with my glider's balance?

One more thing, mind the airflow direction on the underside. If a feather sits proud of the surface at an angle it can buzz or flicker at certain speeds, which looks alive from the ground but gets old fast. Lay them flat and pointed back like they'd sit on a real wing and you're fine.

Not sure I buy the 'lay them flat and you're fine' part. I glued some mallard feathers on a scale build a couple years back, laid flat and swept back just like that, and the quill ends still caught air and lifted off the surface after a few flights. Foam board glue doesn't hold a hollow quill shaft that well once it flexes in the airflow. I ended up pinning each one down along the shaft with a thin strip, not just the base, before they stayed put. Also they curl like crazy in morning dew, so test on a dry day.

I did a feathered hawk build a while back and the CG worry turned out to be nothing. A few grams at the trailing edge on a slow floater just isn't enough to notice, but check the balance point after everything's glued on anyway, before the first toss. What actually got me was durability. Goose feathers held up okay, turkey ones went stringy after a couple of rough grass landings. The light mist of matte did help stiffen them without adding much, just keep it to one pass. I'd also skip the very tips of the wing where it always hits first.

On the clear coat question, go easy with it. I did two full passes of matte on a feathered owl build and it added almost nothing to the weight but turned the barbs brittle, so instead of going stringy they just snapped clean off at the first hard landing. One light pass from further out kept enough flex in them. Also worth test floating a feather with spray on it before you commit the whole wing, some soak the mist in and get heavier than you'd expect.

Update: good point about the quill shafts not sticking to foam board glue. I tested a couple scraps last night and sure enough the glue just skins over on the hollow part. Thinking about cutting a thin slit into the foam and sinking the shaft end inside with a dab of glue, so it's mechanically held instead of surface glued. Anyone tried that, or does the slit weaken the trailing edge too much?