If you're asking, here's what I would do. I would build the business case for the app. Sure, make a demo. Get feedback. Push it until it's clear there's a strong demand for the product, rather than an assumption that people would want it.

A good CTO is more than knowing Swift and if you ever have a technical startup, you're simply going to have to have a good CTO who is deeply technical. It's just a must. But you can honestly get most of the way there just by validating the concept so well that potential co-founders will feel like a fool not to sign on.

Also, you don't need great chemistry with a co-founder -- just mutual respect and open communication.