No. You should build with iOS 27 SDK, which is included in the Xcode 27 Beta, that you can download from Downloads section of dev portal

I've noticed a lot of swift-frontend process crashes and Xcode failing to do basic operation, like search for callees. Not sure if it's related, but it's annoying for sure

We're having the same issue. Were you able to find a solution for this?

NO. We use Xcode 12.5.1 to build enterprise apps and they run fine on iOS 15. Make sure that your build machine is running at least macOS Big Sur (11.6) though