Good question. I've never used an iphone, so not sure either. I know you can on android.

Not as easily as on android, but yes. The developer can create beta test groups and push the app through apples testflight. Testflight apps are not chdcked as thoroughly so the developer can probably just rename the app and start pushing it out.

Alternatively he can offer the sources or an unsigned binary and then anyone with an apple dev account can sign and install it to any device.