Seems like just about any clue that I can think of off the top of my head that there might be something romantic going on, is actually just ambiguous enough that Namori might be tricking us into reading whatever we want into what we see and read.
There might be an entirely innocent way to read between the lines of these clues instead.
For example, in today's comic, we're talking about a compliment - it doesn't have to be a romantic compliment, and indeed, it's apparently a compliment about Megumi's service at work. Which is sort of a boring way to read the story, that Nadeshiko is simply complimenting her friend, but then, Nadeshiko wasn't the ONLY person we know was there to hear the compliment: if Nadeshiko was telling the truth - and I have no reason to think she wasn't - then at least one of her sisters was there, too, which changes things a little: now we get Megumi and Nadeshiko withholding just enough details about an innocent compliment to troll their friends into thinking something romantic is happening instead, which wouldn't be far off from what Namori is doing to us, if this scenario is accurate.
And we've gotten more direct hints from time to time that there are friendly relationships between the characters of different age groups happening mostly off-screen, such as Sakurako apparently talking to Misaki a few times after the misunderstanding about thinking she was dog at the park, and apparently being on pretty good terms with her. And, Nadeshiko seemsto get on pretty well with Himawari when Sakurako isn't there, even teasing her about Sakurako from time to time.
It would allow Namori to take the clues into some really surreal-sounding directions that don't really need to all point the same direction, and keep us guessing for a very long time about what's really happening between the lines.
So, besides an "only one girlfriend per character" solution, and "Nadeshiko is dating everyone" solution, there's the "None of these characters are dating, and we're being trolled" possibility. And somehow, any of those possibilities sound like Namori! Somehow, I get the feeling we'll never get a clear answer, just because leaving it up to our imagination to think of something sillier than Namori could have imagined for us is all part of the fun.
I'm not committed to the theory or anything, but I have tried reading last several chapters I've seen this way, and the stories take on a very different dimension when I assume that one or more of the other, younger girls might be interacting with Nadeshiko and her friends in entirely innocent ways, and sometimes the jokes are even funnier that way, or funny in a couple different ways when subsituting different characters and so on in between the lines of the clues we get.
It's left me wondering what exactly really happened in the subplot where Nadeshiko's "girlfriend" appeared to have broken up with her for a couple days!
Anyway, if there's only one girlfriend, then Nadeshiko x Megumi looks most likely. But, I don't think we'll ever get a clear answer, and I expect that the clues that we get will always be open-ended enough to allow for more surreal "joke" alternatives, including "all of the above" and "none of the above"!