What's the best way to distribute an app only for some devices?

I have created an iPad app that I need to be installed in some devices around the US. I have developed a micro MDM to supervised and manage the devices. Through my MDM I can install new versions of app, restart the device and see a lot of information, like status, name, space available, etc...

Currently, I am building the app using Xcode as a Testing/Distribution, but that are some problems with that.

1- I need to turn on the Develop Mode on every iPad. As they are in different cities, it's not an easiest way to handle.

2- I need to add all devices on my development Apple account (not only on my Business Apple Manager), which causes in other two problems.

2.1 - Apple asks for 24 to 72 hours to add devices to my account

2.2 - Apple has a limit of 100 devices only per development account

2.2.1 - You can delete and/or replace devices once yearly only.

I believe the best solution would be the Enterprise, but I want to know how to proceed correctly with that.