I think you mean 'collapsible'. By running the nouns together like that, the idea is that it is something that can be set up as any of those things, or possibly a combination of them.
i.e. it could be a boat, it could be a raft, or it could be a tent. Or perhaps it is a boat or raft, with a tent on top. It's not clear exactly how it works, but it also probably isn't important to the writer, as they didn't go into details.
The second part means that if Kilner had thought about the design more thoroughly beforehand, he could have redesigned the contraption in such a way that it could have been made into a glider as well.