Not the jam organizer but I think mouse should be allowed as long as keyboard is the default. It shouldn’t hurt anyone to have mouse allowed, if keyboard is the default and works flawlessly.

Here are some blind-accessible games that use keyboard and/or controller to apply projectile mechanics, which may help inspire you:

Fourfold Onslaught: fully blind-accessible FPS: https://itch.io/jam/games-for-blind-gamers-5/rate/4339707

Circular Bowling: “Shooting” mechanic to shoot at bowling pins: https://itch.io/jam/games-for-blind-gamers-5/rate/4339924

Maaaaybe, Spinning Turret: https://itch.io/jam/games-for-blind-gamers-5/rate/4324825

And last but absolutely not the least, Valencia Tales, winner of the blind judges committee and 2nd place in the overall community ranking in our jam. It has very accessible navigation and lock on mechanics that everyone should absolutely steal and apply in their games, and I think can work for FPS: https://itch.io/jam/games-for-blind-gamers-5/rate/4274488

I hope this helps visualizing how an FPS can work with a keyboard, as well as being blind-accessible.