Do they say that was a summer festival in that chapter? There are other festivals that are during the Japanese school year, including a couple in spring, to say nothing of possible local events. Festival activities are kind of the same for a lot of festivals, so unless they specifically called it out as a summer festival, I think there's wiggle room there. Very good chance there's other contradictions, though.
At least the translation doesn't explicitly call it a summer festival, but I doubt people wear yukata for festivals in early spring.
It mainly depends on the weather, from what I understand. If its warm enough, people will wear yukata for a spring festival.
There's another good one with HonoEli. Honoka went to Eli's home for the first time on Christmas. Soon after there's a chapter where they're celebrating Halloween together at her home.
Yeah, I definitely think the Halloween one would seem to come after the Christmas one. If you really want to make it work, the Christmas one doesn't explicitly say Honoka has never been to Eli's, just that she hasn't met Ed and Ian yet. She's freaking out, but that might more be because of the prospect of a party than the pressure of going to her house for the first time. She is obviously assuming that there will be people there besides them and the host family. And you don't see them interact with Ed and Ian during that scene, the Ed x Ian scene after could be later.
But it's a big stretch. They definitely seem much closer in the Halloween one, it's clearly meant to be later in their relationship.