Well, she has said that M won’t really change at all (as in personality and who he is). He will just get less hostile, lol.
As for the clingy part, I think that clingyness, like romance, might be defined differently. There’s this little gem:
Early in the relationship (and I’m assuming there’s actually a relationship instead of it being during the flirting or banging stage), M literally doesn’t give a damn about the MC enough to bother trying to get them back if they walk (and would immediately look for someone else). But deep into the relationship, M would “slink back” because they wouldn’t want to lose the MC.
That isn’t changing M’s personality, it’s just that M actually feels something so they try to fix things. So I think that, while M won’t change, they just behave differently once they actually care about the MC past “I want in your pants.”
It’ll take a really long time to get there, but once it’s there, maybe the clingyness and other stuff would be more obvious. As @resuri08 said, M is all about action, not words. They may not show affection in the way N or F do, but they will show it. And, who knows, maybe @Mewsly is right and Sera will decide to make M gushy and lovey. Given the things I’ve seen in the asks, I won’t hold my breath, but it’s possible Sera will change her mind. At the very least, what we get from M will be whatever the character can offer to show their feelings, just like the others.
To @Mewsly I would so bet you that N-mancers get a kiss way before M, at this point (and N will likely kiss the MC at the carnival or at least by the end of the book). I don’t think M-mancers will get a kiss, much less a makeout session before the end of book 2. After the ‘long game’ comment from M’s thoughts, I get the feeling that MC’s who want to have some unfettered fun with M are gonna have to wait for a long while. It may be that Sera intends for us to wait until F and N get into a relationship with the MC before our MCs can have a go at M. So yeah, I’d totally bet you an Amazon gift card on what you said!