Waves is also a good song. Looks like Robin works with alot of people.
Yes, he remixes a lot of other artists like Mr Probz
Was Prayer in C his song or a remix of someone else? 
It was originally by Lilly Wood and the Prick
Is there anyone recently he has been working with that I would know?
He worked with  Ilsey and Francesco Yates on his second record
It looks like he had a clothing line at one point. Is that still around?
Maybe, it seems like it was called R3HAB?
I don't think so. What I found was a mention of Q/S Designed by Robin Schulz. All that was given was the name.
It was probably successful, he's supposed to be a down to earth guy