...
- if a swarm fails and the swarming queen is no longer present; are they more likely to take off with a hatched virgin? Do they still have the swarming fever?
If the original reason for swarming was over crowding, then YES they will still want to swarm with the first virgin queen to hatch!
- is cast dependent upon the number of bees left after the prime swarm? So, the bigger the colony the more likely?
See above answer
- do you get cast swarms on the early swarms that go or is it more common later in the year?
Personally I would say that if there is a prime swarm then there could be cast swarm(s) regardless of the time of year.
- if weather prevents virgins taking a cast will the pressure ease and she's more likely to settle down - kill any sisters - and then go on a mating flight when the weather improves?
Not sure that this will be the case, it's also not always true that swarms will not come out of the hive in bad weather! I had a swarm hanging on the side of the hive that it came from on a cool and damp day the other year!