I think the best course here is to tech IW next, Writing is not going to help you at this stage when you'll still be expanding and you lack the food surplus in Bibracte anyhow to take advantage of it by running specialists. Sure the research bonus to palace with gold is fine enough but you're not creative either and it simply is not worth the cost at the moment.
You're definitely going to want Gallic Warriors if you attack Mansa in any case, skirmishers are tough and he will have that copper hooked up before you can hit him even on epic speed since this is immortal, pillage the copper then sacrifice a big chunk of your units to take Timbuktu, after that you will have an easier time of mopping up other cities with the gallics.
I would forego the pig as you suggested and instead get the gems up and running asap. Also send out a WB first chance you get to find SB's land and begin trading with him for the intercontinental bonus. With this strategy the best case scenario goes:
a) Gems asap for good commerce boost. Establish trade with SB for best trade routes.
b) Mansa spreads islam to Lincoln and/or builds shrine (even if he doesn't you can simply run a priest).
c) You conquer Mansa and spread islam to SB, then you lead the muslim block against the confu block with lots of cash from the shrine.
Be aware though that SB likes to found religions and will try to get one so it's not certain that he could be converted to islam.
I agree with you on putting off the pig city and immediately flip-flop my position. While blocking the terrain would be nice, the extra cost from a 4th city plus the time needed to actually get it to a point where it could contribute to the war effort might be too much. Plus garrisoning it means units not used for attacking.
I don't think the losses taking Timbuktu are going to be as bad as everyone thinks. Gallics only need 1xp to get G3 which greatly reduces your initial losses against the healthier defenders. As a Holy city/capital Timbuktu is going to have 40% culture at the least. Possibly 60%. But the 50% retreat odds for G3 plus the free cover and C1 means that your losses will be greatly reduced.