When Amazon added Fire TV capabilities to the Echo Show 15, it went out of its way to block the ability to install apps from unknown sources, a.k.a. sideloading apps, through the traditional Fire TV methods. With the ability to make ADB connections to the Echo Show 15 and the ability to grant permission for apps from unknown sources both missing from the device, it seemed like there was no way to sideload apps. However, it appears as though Amazon left a loophole in place that can be used to sideload apps in a roundabout way.
Full guide for How to Sideload Apps on the Echo Show 15 can be found here.
While there is no way to grant an app, like my Downloader app, the ability to install APK files from unknown sources, AFTVnews reader Super Street Racing discovered that one of the default android system apps on the Echo Show 15 already has the ability enabled by default. Installed by default on the Echo Show 15 is Google’s File Manager app. It is among many default Android system apps that are present on the Echo Show 15 but have been hidden by Amazon as they generally serve no purpose for Echo Show 15 users.
It turns out you can use another app, like one of the many file/app managers available in the Amazon Appstore, to launch the default Android File Manager. When you do, you’ll find that it, by default, has permission to access the device’s Download folder and install APK files from that directory. So, one just needs to get an APK file into the folder using a download manager or file explorer from the Amazon Appstore and then use the default file manager to install it.
I’m guessing that leaving this behavior present on the Echo Show 15 was not done intentionally by Amazon, given that traditional sideloading methods were removed. I expect that a future software update for the Echo Show 15 will patch this sideloading workaround. Until then, enjoy being able to install any APK you want on the Echo Show 15.