I'm not a Ronalsway controller so cannot answer specifically about their operations but in generally speaking the answers would be:
1: I'm not sure exactly what you mean by how "the flight is handled", but from a controller's point of view whether you are VFR or IFR receiving a RIS you would be treated the same way i.e. a RIS is a standard level of service irrespective of flight rules.
2: If you have not received a clearance ask for one (approx. 10 mins before the boundary of CAS). Either Shawbury would have asked you to 'freecall' Ronaldsway radar or if they weren't too busy would have passed on your details and may obtain a clearance from EGNS.
Never assume coordination has taken place between controllers or a clearance has been granted, always question it.
3: Yes just ask for the IFR clearance. However if EGNS radar is busy then they may just tell you to remain clear or delay the clearance until traffic eases.
4: In class D airspace controllers have to
(a) separate IFR flights from other IFR flights
(b) pass traffic information to IFR flights on VFR flights and give
traffic avoidance advice if requested
(c) pass traffic information to VFR flights on IFR flights and other
VFR flights
In order to aid separation, flights will often be cleared at whole flight levels when IFR (unless they being vectored for approach) i.e. using the semi-circular rule.