"Switch to mobile data" setting - Will apps configured to do updates etc while in wi-fi only mode use mobile data if wi-fi is down? I have a number of apps that have settings to require a wi-fi connection in order to synch/update/download and I have them configured as such.  What I'm wondering is if I have wi-fi on and configured to switch to mobile-data when the wi-fi connection is lost and now the only connection I have is via mobile data, from the Android/app perspective, is this still considered wi-fi mode and said apps will update using mobile data?
Depends on the app.

It is possible, for the app developer/creator, to check if you are using wi-fi, and it is also possible to enforce the app to only use this kind of connection. However, there can be apps that are not well made that doesn't make these checks. Usually, if they have this configuration, they will use only wi-fi (if that's the way you set).

The best way to check if the app you are using is reliable is reading user comments of each app - usually, if one user notices the app was supposed to use wi-fi only and it doesn't, he posts a complaining "one star" comment there.