There is honestly no bad months in Chidenguele. It fishes well all year round, when game fish are quiet, bottom fishing is tops. One year may be different to the next. It is probably a factor of what kind of weather you prefer that may be the decider as to when you should go. In general it is advisable to go for a longer period once a year rather than a few short trips. Nothing worse than hitting a few bad days on arriving and having to come home just as things change and the fishing gets wild!
Speak to Greg Holwill, his contact should be on his nhambavalelodge website. I am sure he will give you expert advice.
Cheers