I can't imagine that one-handed idea happening, doubling the number of shotgun animations isn't really something that happens in Underrail. Styg would rather implement an entirely new weapon class with that animation budget. That's just the way things are with spritesheets. Well, maybe they could just add one-handed shotgun firing to riot gear sprites.
Jetski mechanics are pretty much as Styg outlined them in the jetski devlog. Stability is important consideration in naval combat and there are some ways to alter it other than just switching to entirely different jetski. Having stability bonuses on weapons seems plausible, maybe there already is something for that. Might be cool to have handheld weapons specialized for naval combat in the trunk of your jetski, but on the other hand swapping them around would get old after a while.
I did think about the spritesheet issue, yeah...
Basically I thought it wouldn't be a big deal because Shield Riot Gear has very few sprites anyway, you can only use the knife and the pistol with it, no SMG, no rifle, no sniper rifle, no sledge etc. So basically you would add just one weapon sprite set for the "one-handed long gun" weapon for Shield Riot Gear only, which could turn out to be pretty versatile and re-usable in the long run. No changes to the other spritesheets.
There is also another way which requires no extra sprites, which is to simply use the Riot Gear pistol graphic for one-handed shotguns.