Yes, Alexis should show up regardless of relationship status, she'd be going to visit her friend not for a date after all, i feel like that's a no brainer.Fair points you made. So even if player (MC) isn't into Lexi it would only be natural for her to tag along. She is after all bf with Erynn. Melissa also fits in perfectly, no need to explain why. John and Iris could make sense but it's more 50 - 50. If everything plays well they could jump in, perhapse even rekindle old flame between Iris and MC or Melissa and John. Also if player is on Melissa route it could make them close. Options are wide and great.
Maybe extra icy dialogue if you got the Layla vs Alexis relationship end
Iris isn't obligatory, but like i said before easy to buy in.
Keep in mind that "Iris no John" doesn't change anything in terms of family reveal, she'd be there just as support (and probably more teasing of their special bond).
To have any kind of family breakthrough we would need Melissa to recognize John or something along those lines, so maybe he visits after his daughter tells him about "the girl who got beat" more towards the end of the episode (and here we have another cliffhanger).