This is a long shot reason, but if your app uses any google sdk, like TTS or maps, etc, then that might be why Amazon's store is saying that there is no supported devices because Amazon devices do not have google services installed on them, so any app needing google services will not run on amazon devices.
It looks like their upload checker has a bug in it and won't accept an updated apk that includes a LOWER minSDK then the current version in the store.
But I am not convinced the minSDK of 14 is your problem without seeing that post you mentioned.
As a test to see if the new apk will give you more devices with the minSDK of 5, just create a new product in amazon's app store, rename the package name of the apk, and upload it to see if it gives you more devices. (you can always delete this "test" app from the amazon app store)
Do Amazon Fire devices support Google Pay?