SM
If you're not Nigerian, there are no expat copilots in Nigeria. If a copilot is needed a Nigerian is employed. There are often two expat Captains flying together though. If you don't have the qualifications for Captaincy (even if you're not flying in Command) you won't get employed. These qualifications are normally determined by the oil companies Bristow is flying for. Contract is 7 weeks on 5 weeks off, pay around £42K. Accomodation depends which contract you're on. Maintenance is normally good after going thru a bad patch some years ago (see various remarks by SASless on threads here over the last few years). The main thing to prepare yourself for is not Bristow but Nigeria. It's different from a lot of places in Africa and people tend to have very polarised views about it, so if you have any friends there talk to them beforehand if you're thinking of going.
JW
Why would you think CHC will wrap up all the Nigerian work? They may be about to take over ACN, but it's no different from the situation in the North Sea or GOM. The oil companies want to keep at least 2 operators around to keep competition up and prices down. CHC pay less than Bristow, though their work schedule is 6 weeks, 6 weeks.