It seems this game would be most fun in multiplayer. The store page says it's single player. Are there any multiplayer modes available?
Online? LAN? Local Co-op?

You can play in local Co-op by pressing both triggers of player 2 gamepad when starting a level [both players might need to press both buttons at the same time, IDK.]

There's online multiplayer for up to 8 people, but I'm trying to figure out how and where that works myself right now. Anyone?

Thanks, that's at least something. Online multiplayer would be good to have indeed.

Yeah, apparently that's in an update that GoG still doesn't have, after ~1 month. Then again, I've been hearing stuff about unstable games after the last update, especially with multiplayer in use, so maybe they're just trying to fix that with their steam "focus group" before delivering that update on GoG...?

I'll try to get an update on that situation. Is the local multiplayer split-screen? One left, one right or top/bottom?

Left / right, from what I've tested. Dunno if you could, or should, change that to horizontal, because you'd rather see ahead and up / down than have a wide angle in this game. I assume.