I'm happy to see the plethora of information you're gathering, I hope it's helping you

It's overwhelming me though, so I'm just going to stick with the questions you specifically asked me:

2) Yeah, I think your archipelagos would have to be volcanic if you are going with the young world hypothesis. But ... your islanders will actually have a love/hate relationship with them, because while a volcano can be incredibly destructive, it's also EXTREMELY good for life, making the best soil in the world. For example, Hawai'ians both revere and fear their volcano goddess for both bringing and taking life (I'm oversimplifying a bit here, so if anyone knows more about that topic please feel free to elaborate).

3) Developing plate tectonics is simple, but not easy There are several threads that have gone through the process if you peruse the world/regional wip forum. However, keep in mind that a young world (if you do decide to go that way) will have some crazy plate tectonics. If it's extremely active, there will be a lot of little plates that haven't yet merged the way the ones on earth have (this is my conjecture, so I may be wrong. It just seems logical to me).

4) Actually, yeah. jbgibson's hypothesis sounds reasonable enough to me (for a novel, it doesn't necessarily have to be completely realistic, just somewhat believable. Witness the years-long and unpredictable nature of the seasons in a Song of Ice and Fire).

5) For the water cycle, I should have pointed it out to you before but neglected to do so. Check out the How To Get Your Rivers in the Right Place sticky in the tutorial forum. It's incredibly helpful to understand rivers and lakes.

One other point that you had asked earlier ... you asked about humans on a young world. This is the weak link in the young world theory; you mention that there's no magic, but is there high science, maybe even high science that isn't there anymore? Because you need a way to seed them, they won't have developed naturally. Maybe they're a colony and terraformed the world but then regressed. This may also help with the cloud cover thing. I write fantasy, so I'd use a Gate