1. I have used -uś with Robert - Robuś - this doesn't seem to be a name that has a diminuitive or many of them. Is there a better one?
Robert, unfortunately, doesn't have a good diminutive in Polish. Don't use Robuś unless it's your son.
3. What about a name like Jarek?
Stay with that one. Using Jaruś or Jareczek would be too personal.