If JOTA provide an aircraft it will cost them. Their business model, like much of Titan's is to come in and lease when needed by other airlines and specialist charters- at a price. They could I guess set up an operation to do it for Stobart with a different model - certainty of operation at Stobart's risk. There are plenty of RJ85s about- five currently at Cranfield. I guess that would also be ok for the franchise- not sure Flybe would want their name on some small Eastern European operators jets (e.g. SmartLynx)
Dusseldorf is in ideal ATR territory and would be more efficiently served by a turboprop than a jet. Some slot issues there though so may be Dortmund or Munchen-Gladbach