Just give the editor a long list. Don't try to predict who takes a summer holiday. It's absolutely not your job to know people's schedules. And definitely don't try to research that or contact potential reviewers.
Also, while it's fine that you assist with a list, it's really the editor/journal's job to get the paper reviewed. I would be a bit irked to have made a requested submission (they actually reached out to you) and then they can't even get the paper reviewed. Push comes to shove, just move the paper. Might not be summer or your topic being obscure, may just be a not very efficient journal/editor.
Furthermore, I find it strange the idea that the whole season of summer precludes reviewing. I bet PLENTY of papers get reviewed in the summer. Sure, people go for vacations, but in the US at least, tenured professors (and I would assume some US tenured R1 profs could be in your superstar list), don't usually take the whole season off. If anything, many professors I know have more time in the summer than in the academic year. In any case, even if there's less, the wheels still churn and lots of papers get reviewed. This editor also ought to feel some responsibility for all the non-summer time when the paper sat. But really, I think you are overthinking it with worrying about summer versus reviewing.
If the editor is doing his job, he will only leave the paper with people that say they can review it (if no response or a "no", move on down the list). Not just dump it into inboxes of 3 people who are spending the summer on an Alaskan field trip. But even this, I would feel is just the editor doing the editor's job. You shouldn't have to micromanage him. Just move the paper if you feel he's not being efficient.