Marcel will play doubles with Marc López at the London Olympics
He wasn't selected to play singles, but good to see he'll be able to play the Olympics with his regular partner in the ATP tour.
Sonja, I'm not sure that rule about Davis Cup is true. Two examples are Donald Young, who is playing the Olympics while has never played DC with USA, and Ryan Harrison, who made his DC debut this year, just like Marc.
The key is you make yourself AVAILABLE for selection.