MC doesn't start warming up to Erynn until after she wakes him up, everything prior to that screams avoidance and resentment.
If MC told Mel that he didn't want to see Erynn and she was the reason he left, she'd straight up tell Erynn he was back, which he wasn't ready for, or even sure he wanted.
You can chose to stay with her, and have a nice conversation right away... that's on you, how you play the character.
His initial reaction can be put down to having a "dear in the headlights" moment, he saw Erynn and didn't know how to react at that moment.
The way i see things he was split between wanting to be friends again and his resentment... all the way until the park conversation.
I was referring to the fact that if you go running with Layla instead of going out with Lexi you can continue Erynn's route in parallel with Lexi's
I think you are wrong here...
It goes:
Ethan rescue> Talk about Alexis or not...
If you chose to tell Erynn things stop immediately.
If you don't... blueballs scene

There's almost sex, but she doesn't know you are still seeing Alexis(because you decided not to say anything).
After that we have Alexis date or Layla training, by chosing Layla training you avoid Erynn+Alexis meeting at your place, nothing happens here, she just drives you to school.
Next meeting is after TC when Erynn stops by... if you haven't mentioned Alexis before the MC does it here automatically... all Erynn scenes going forward became their toned down versions.
Now, Erynn is, or can be depending on playstyle, secretly still in love with the MC... but she hasn't done anything that actively crosses the line into cheating on her friend.