Will it be possible (in the future) to have imps connect to a wireless ad-hoc network?
Ad-hoc isn’t so good for power, and generally most devices that could do traffic routing have AP mode these days (as it’s supported by the chipsets) - what scenario were you wanting to implement?
Nothing in particular, was just hoping to get some dev time in, plus thought maybe it might be useful in some situation yet to be thought of. 
I’m not terribly familiar with wireless networking protocols. How can I find out whether my Android device is using ad-hoc or AP mode when it provides a tethering connection? Is there a particular Android app + settings you can recommend that are known to work?
Pretty much any device offering “hotspot” is AP mode (ie, hotspot built into the OS). Windows and MacOS will both list ad-hoc mode networks they find in a different way - easy to see.
how much more power drain is ad-hoc? I am also assuming since no ad-hoc support, WiFi Direct is out of the picture as well. So Electric Imp devices can only be supported in AP Infrastructure mode. Is this correct?
With ad-hoc the imp can never use low power mode; this is about a 16x power saving when enabled and no traffic is flowing.
AP infrastructure is the only supported mode, yes.