I suspect the two biggest issues will be range and maintenance support. Required runway length for the RJ85 on a ISA 30 day (89F) looks to be around 5000 feet with a full load of passengers and standard bag weights to fly a 1000 mile segment. Actual bag weights I suspect will be higher with all the dive gear etc...
The runway available is 4645. They will have to pull a bunch of bags or people unless they go to SJU for gas. If thunderstorms in MIA require a alternate airport then they will need additional fuel meaning even less payload. This does not even factor in if the runway is wet. If a rain shower soaks the runway after the aircraft is fueled they will have to wait for it to dry before takeoff leading to delays.
The maximum altitude on the AVRO is 35,000 feet which means you will be below the tops of thunderstorms in route and may need contingency fuel for diversions.
On the return leg from MIA if it's one of the rare days the airport is IFR the flight will have to divert to SJU.
On the subject of spares the first few months they will have a spare however they plan other flights so it will later be in use. In addition the likelyhood of them flying a spare from EIS to MIA to cover a broken aircraft in MIA is slim. It's simply too expensive. It would be cheaper to rebook on other airlines. Lack of a interline agreement does not mean they can't rebook you. They simply have to pay the other airline with a check when they walk you over. It's done all the time.
G