How to stop iOS 10 alarm from silencing itself after 15 minutes?
The iOS 10 alarm clock will turn off after 15 minutes and there's nothing you can do about it, it's built that way.  Even if you turn off Snooze, it will still stop.

If you need something else, and it looks like you do, I recommend going to the SE Software Recommendation site and ask for other options.  I'm sure you're not the only one who needs alternatives because of this change.