I think I missed quite a lot, as it's really difficult to scoop an entire batch of somen in one go... for me at least. But the whole thing was filling enough for me, despite the lack of side dishes.
Yes, unfortunately, you're right. It's only available during summer.
The flume is shared by several diners. I think yes, there'd be people who mind it, but judging by the number of customers, most wouldn't.