You are engagement frozen for five training years when you start at BA, however this can be reduced to four years for a first command. Unless you are short haul it is very unlikely you will be released from your freeze early. I believe there was a supplementary bid last year due to a shortage of applicants for Gatwick commands and some pilots were released from long haul freezes early. However this is most certainly the exception rather than the rule, and was in part due to the increased flying, which arose from the purchase of Monarch slots. It is unlikely to happen again.
As far as failing a command after returning from long haul. BA are pretty brutal nowadays and my understanding is that you would remain in short haul in the rhs, with the possibility that you could try again when you are no longer cat c (2yrs).
Recruitment is expected to number anything upto 400 next year, however BA recruited huge numbers in 2014/15 and many of these will be unfrozen and will take the majority of the long haul positions. This has not been the case in previous years as recruitment was quieter between 2012-14. My guess is that DEP short haul is more likely going forward.