Just for completion, after taking the trip I can finally answer my own question - yes, the bus ticket counter in arrivals accepts cards.

The bus is okay, 717 goes near the Souq Waqif/Museum of Islamic Art without too many stops so I imagine it takes comparable time to Uber. It's a bit more tricky on the way back - the bus stops are quite 'bare' and have no information printed whatsoever - including which buses operate from a given stop. I ended up standing on a bus stop for 20 minutes hoping it's the right one (it was).