Nene agreed to play up her romance with Youko to get Sumika angry/jealous, of course Sumika finding them kissing was intentional. Just because later on Nene decided she wanted to be with Youko for real doesn't mean she wasn't a willing participant in the charade from the start.
All I saw from their date was that Youko hurt Nene by not wanting the same things Nene wanted, and being overall oblivious and insensitive towards Nene's feelings. And sure it sucks for Nene, but she wasn't manipulated into sending Youko a message or into sleeping with her, those were all her (in hindsight) bad choices.
Nene agreed to do it, that's true, but it was all Youko's idea to begin with. And in my opinion she did it just to have fun, not to really help her, or do you get the impression that she truly cares about Nene? I would love to be wrong, but I find it hard to believe that she does. Youko didn't even contact her again after she left.
I don't think it being Youko's idea paints her in a worse light than Nene since she still chose to go with it.
Yeah, I think so. Don't get me wrong, seeing Sumika losing her mind probably was fun for Youko, maybe a little bit for Nene too, but I think she is being honest when she says the whole thing was for Nene's sake. Otherwise she wouldn't bother telling Nene they are just playing romance against Sumika again, she'll just enjoy Sumika's reactions while having fun with Nene.
I'll wait and see how her suddenly abandoning the café is explained before I make any judgements. After all this is "miscommunication, repressed feelings and worst possible asumptions" the manga.