1- Yes, Melissa should eventually run into John.We'll know when the MC feels ready to meet his biological father. He made a similar comment when talking to Emily.
There are three problems with the assumption as to whether Erynn and Iris are half-siblings:
1. Melissa would have to be present when John picked up Emily
AND!!!
2. After all this time, John AND/OR Melissa should recognize each other. Melissa mentioned in a conversation with the MC that it was a one night stand!!!
AND!!!
3. then who is Iris' mother???
2- Dude, Melissa still has day dreams about John, you think she wouldn't take one look and immediately click?
And John, he takes one look at Erynn and says "you remind me of someone i knew from my youth"(not an exact quote, just the spirit).
3- After banging Melissa, John had to move to another town, presumably in this other town he got a girl pregnant and that's where Iris comes from.
That's pretty good lol.a woman never forgets the appearance of the man who put a roast in her oven.
And i agree with the rest of your post, Melissa still dreams about her date with John and John's mind immediately thinks of Melissa after taking one quick look at Erynn(must've been one hell of lay).