Chris: Guinea visa can be done in Dakar and you have to go to Dakar to get your carnet stamped anyway.
It would be great if you could get a Cote d'ivoire visa beforehand. We went to Conakry to get ours but if you already have it you can just ride from border to border and don't need to go to Conakry to get the Guinea visa page put in your passport. You can then cross the country in less than 5 days on the entry paper. I'm not sure where you could get the Cote d'ivoire visa though as the embassy in Dakar won't issue it. Maybe try in Morocco?
We got the Cameroon visa in Abidjan but it took 8 days to process. I'd rather spend time in Abidjan than Lagos! Again, not sure where else you could get that but yes, you want to avoid Lagos if possible.
We haven't received our Congo visas yet which is why we're still in Douala.

Problem with getting lots of visas before leaving your home country is the expiry of the visas you have already obtained. If visa processing takes up to three weeks in London for a Ghana visa you can't afford to then send your passport to multiple different embassies after that without the first one you obtained expiring! Expediting our Ghana and Nigeria visas in London meant we spent about £750 on those two visas alone.

Good job on the passport, you'll need loads of blank pages!

Let us know if you need anything else.