I think it is a no brainer! what is it abotu the Gaelic speaking bit that is holding your wife back. If you had both schools English speaking and had a choice you would choose the better school? Why is it different that they speak Gaelic? I studied bilingualism and child language acquisition at university. It isn’t a problem you and the wife not speaking Gaelic, s/he will go through a period of not speaking either language very well and there will be a period of time where their linguistic development will be behind but that is because the brain is struggling with the 2 languages, grammar building, etc however they’ll hti about 7 and be fluent in 2 languages, way ahead of their peers.
being able to speak more than 1 language is hugely advantageous (not matter what the 2nd language is) and will make it easier to learn additional languages. That has all been proven in studies.
I wouldn’t worry about kids being “ousted” as they go to an “odd school” kids will always find something to bully/pick on with other kids and if this is all they can find then you’ll be fine.
I didn’t go to a Welsh secondary school and forgot my Welsh I do wish my parents had further pushed it as kids. I ALWAYs get asked “Can you speak Welsh?”, “Can you say xxx?” and I wish I could. I also always wanted to get into the media, editing on TV, camera work etc but it’s a crazy business. If I spoke Welsh I could’ve easily got onto S4C always desperate for Welsh speakers!! See advantages.
No brainer .. Gaelic school.