I'm not here to make the argument about which of cheating or neglect is more hurtful. I won't impose my own perceptions of pain, especially when it's not me under such circumstances.
On nanase being selfish, this is merely an observation that many of her decisions benefit herself alone. Example: 1 she ditches yuni in favour of club on their anniversary, 2 the festival date where she invites her teammates to hide the fact that it was supposed to be a date, 3 when she asks yuni to cheer her on in Osaka without considering how difficult it may be. Mind that in all of these instances fuuko manages to insert herself, it's exactly WHY she can insert herself.
I won't argue when or if cheating is excusable, forgiveable or otherwise. Everyone can make their own decisions on it.