"Open with" in explorer will not change the file association and I believe the checkbox you are referring to has disappeared as of build 15063.
In the Default App panel, if I go into Photo Viewer and click choose a default, I will only see Paint, Pain 3D and "Look for an app in the Store". No choice for a dialog with a complete list of app. Again you are probably referring to an earlier version of Windows 10.
> I believe the checkbox you are referring to has disappeared as of build 15063.
I'm on build 15063, and it is there.
> In the Default App panel, if I go into Photo Viewer and click choose a default, I will only see Paint, Pain 3D and "Look for an app in the Store".
It shows the apps that are registered as photo viewers. The fact that IrfanView doesn't show up there means that it doesn't do that. For me, that dialog lists Paint.NET and other non-Store apps. Again, this is on build 15063.
Indeed, even per your description, it lists at least one non-Store app - that being Paint. So it's clearly not locked to the Store.
Now, I don't know exactly what it takes for the app to be considered a "photo viewer", but it seems that you need to have it registered (not as a default, but just generally as a handler) for .jpg and .png? I googled a little bit, and the discussion here indicates that after people did that association in IrfanView settings, it did show up in the app list for Photo Viewer for them:
Might the difference be different localities too? MS has had several large fines in Europe for swinging too hard with their monopoly, perhaps they've changed this option except in Europe? It sounds pretty anti-competitive.
In the Default App panel, if I go into Photo Viewer and click choose a default, I will only see Paint, Pain 3D and "Look for an app in the Store". No choice for a dialog with a complete list of app. Again you are probably referring to an earlier version of Windows 10.