Is it safe to spray matte clear over decals, or am I going to ruin eight months of work?

Go for it. Light passes from farther away than feels natural is exactly right, and honestly the fact that you're already masked off means you're halfway thru the hard part. I've done this on a few builds where the decals sat glossier than the rest of the finish and a matte or satin clear knocked everything down to match nicely.

A few things that made a difference for me. First, thin your coats if you can. Lots of thin passes instead of one or two heavier ones is what prevents fogging and keeps you from burying the panel line wash. I wait til the previous pass looks dry to the touch, maybe 15 or 20 minutes depending on the room, then go again. Also, keep the can or airbrush moving and start your pass off the model and end off the model so you never blast one spot. The decal edges are where a heavy coat will pool, so that first light tack coat matters most.

On the canopy, keep the mask on, but even so I'd spray from a bit farther back than you think you need. If any fogging does sneak under the mask edge, a light polish with a fine grit and some toothpaste or a polish compound will bring it back. If the mask is sealed well you probly won't have an issue at all. As for brand matching, mixing brands is genrally fine as long as the families get along. Water based acrylic clear over acrylic paint, fine. Lacquer clear over fully cured acrylic is usally okay but it can bite enamel washes if you lay it on heavy. If your wash is enamel, that's the one thing I'd be careful about. Test on the underside of a wing or an old scrap first if you have anything similar, just to see how it reacts and how the sheen looks. One more tip, leave the canopy masked until the clear has cured for a day or so, not just dry. Peeling the tape off too early can lift the edge of a fresh coat. Then get that thing on the shelf. Eight months of weekends deserves the display.

Gotta push back on the toothpaste bit. Toothpaste is an abrasive meant for teeth, not for a canopy that took you eight months to get perfect. I tried that trick years back on a 1/72 Spitfire and ended up with fine swirl marks all over the glass that showed up under every light in the room. If fogging gets under the mask, a proper polishing compound made for clear plastic or a gloss dip is the way, not something from the bathroom shelf. Better plan is just sealing that mask edge down good with a burnished edge before you spray anything. Also curious what your wash actually is, enamel or acrylic changes the game here.

One thing I learned the hard way on my last finish: don't judge the sheen under your work light. I thought my final passes looked flat in the garage, then set the model on the windowsill the next morning and the sun picked out every shiny spot. View it under daylight or a plain white bulb before deciding you need one more pass. That extra coat is how you end up milky.

Coming at this from the paper side of the hobby so take it with a grain of salt, but I seal printed cardstock builds all the time and the same nerves apply. My first mist coat ever was on a card model and I held the can way too close out of fear of dust specks, got a couple of wet spots that wrinkled the paper. So the distance thing is real advice, I learned it backwards. Curious though, does vinyl or whatever decal film is made of react to cold spray the same way paper does? Or is that not a concern on styrene?

Update: I went with the light passes from a distance, and the gloss on the decals finally evened out pretty well. It made a big difference. Now I am just being cautious about holding the can too close, making sure I don't cause any of those wet spots that could wrinkle under the layers.