CCA
To some extent nights off before/after is a "that depends" due to a combination of Bidline Rules and EASA. It's usually 3 local nights (i.e. two clear days) before/after, but under EASA the days off before might be controlled by the previous duty (e.g. a SIN/SYD/SIN will mean (I think) 4 days off before).
Nowadays you shouldn't get many roster changes (certainly in Long Haul). It normally now only happens if you lose a rostered trip due to external circumstances ( ATC strike, weather), in which case, roughly speaking, the company have the option of dropping another trip into the days that were occupied by the cancelled trip.
qld330
Does British Airways have any real command training for potential capts?
You'll get a block of formal PICUS sectors a few month before the course, there's also a short ground course (mainly legal stuff). The length of the formal command course (sims and line sectors) depends on whether you are doing a Right to Left on type or a Command conversion (e.g. 747 to A320). Just to give you some idea when I went Right to Left a few years back, on Longhaul, it was 6 sims and 10 line sectors, including the 2 sector final check.
There are failures, can't give a rate off the top of my head but it's not astronomic.
If you do fail it is possible you may not automatically have the chance of another go (and I'd say certainly not within a year), course failures are handled by a formal process and each case is different.