AlpineQuest & OfflineMaps

On a cellphone, tap
https://www.google.com/maps/d/kml?mid=1 ... baKtaK1cME

We see "Open in ..... Offline Maps?" among the choices.

However this gives an unknown error, although it is KMZ.

You need the "KMZ Support" in-app purchase to open KMZ files.
Supported file extensions must be listed in the install file, so there is no way for the app to know at this time if this purchase has been made or not.