I did H20 Glow Night solo last year, and tend to be a little extroverted when at the parks. For Miss Adventure Falls, they allowed you in line as fewer than a party of 3, and I just had to wait until another party of 1 or 2 showed up, which wasn't an issue. For Gangplank Falls, the Cast Member told me tough luck and that's when I stood by the entrance and asked the next party of 2 if they could "adopt me" for the next 15 minutes lol! and they were more than happy to. I will point out that the raft for Gangplank is tight, so I would skip it just cause it wasn't as exciting as Miss Adventure Falls, you had to be proactive in finding a party, and you're VERY close to strangers.
Also, one of the Crush n' Gusher slides is 2-person only, so you can still do Crush n' Gusher, you are just limited to a particular slide.