Maybe underwear just doesn't exist in the Saki universe? It probably is the yuri powers, though that doesn't explain Yuuki who has shown more interest Suga then any women. Also yeah I know none of the characters wear bras but the flat ones don't really need them anyway.
Well, it might not exist, but it may in some form -- it's just almost never worn. Yuuki has that one time from very early in the series where, when dressed as a maid (while Saki and Nodoka were at Mako's, also dressed as maids, and clearly nopan) she faced him and said something about showing him her panchira and lifted her skirt, but he looked away and said 'I don't want to see' or something... but we never see if she actually was wearing underwear. It's the only time in the franchise underwear was ever mentioned, but it's an unreliable narrator (Yuuki...) and it's not actaully seen. The resulting speculation is that panties are only worn for punishment, since she also says that the maid costume was a punishment game.
Also, Yuuki could be bi. Remember that stuff she did with Nodoka, mostly before too, particularly in flashbacks from before Nodoka met Saki, but also in one of the picture-drama OVAs too... so she could get a bit of yuri power from that. It runs out too quickly, though, which might be why she fades after the first half...
I'm sure in the end it'll just be kept as "close friends" for everyone but as you said some of them clearly have a thing for each other. If Ritz does actually make some of the couples official then there is no way that Yumi & Momo wouldn't be one of them. Momo even gets jealous that Hisa and Yumi got close enough to be calling each other by their first names. For not yuri couples I would assume that Yuuki & Suga would get together, in fact I could see that happening even if Ritz doesn't make any other couples official.
Yumi & Momo... this happened in Achiga-hen.
http://abload.de/img/1380421799133sdpnx.jpg (hugging) Was that a "we're just friends" hug?
As for non-yuri couples, that depends on if Yuuki's serious or not. With her, who knows. And that's the only one that's even hinted at in the franchise (Kyoutarou likes Nodoka early on, of course, but they stop mentioning that after a while, and the feelings are most definitely not mutual.)
In general about the relationships, we'll see, though. Ritz clearly doesn't want to actually say for sure "yes they are couples", just leave constant hints that they are. It's worked so far, so I doubt that that formula will change. However, the fans will never forget that one mention (from a Yuuki-Nodoka conversation) about IPS babies...
Poor Mako, she's probably had less panel time than the announcers.
Quite possibly so.
She has a good design, too; green is my favorite color, and glasses are nice. Hisa is definitely the best, but Mako, and as I said also Arata, definitely have been badly ignored way too often. I know there's only so much space, but relatively minor characters have gotten a lot more than either of those ones from "main character" teams...
