I'm pretty sure I've bought shirts with my debit card. I think everywhere else is cash though.
Probably only at the official Coachella store. The problem with taking CC payments is that you need a phone connection. There aren't any phone connections there. The ATM's use a mobile phone system. I'm sure that GV taps into that system for their official store. Other vendors might have to pay a high fee for this.
Do the merch booths sell craft (i.e. micro) beer such as Stone IPA?
The only places open to the general population where they sell any alcohol at all is the Beer Gardens and as mentioned above, they have only sold Heineken. They were plastic cups of approximaely 10 fl oz for $7. And you have to keep those cups in the Beer Garden.
I don't think I'm hosting a 2016 collaborative playlist.