I am in favor of writing as the next tech. Once the economy crashes having scientists is muy bueno. Followed by med->priesthood and CoL. With Gold, Silver and Jumbos Monarchy can be dlayed a bit longer. especially with a Charasmatic leader with the Henge.
Once mecca is absorbed into the great Celtic empire and CoL is discovered, running a spy specialist in Mecca combined with the GW stands a good chance of generating a great spy. Infiltrating Mansa is better than trading with him. A. You will rarely have techs Mansa does not have. B. They are cheaper when ya steal them. C. When you infiltrate an AI they go into espionage panic mode and adjust the slider to try and compensate thus slowing him a little bit. Even with sal's lands, sis is not going to outtech MR Techwhoresupremo.
I list meditation as the path ratrher than Poly because I think rather than settling a GS or an academy. Bulbing Philo is the key. I am concerned that sis may run into the problem he had in the saladin game when the entire continent was religionless. This kept him from running any religious techs. If he can found the only religion on the continent it will autospread to every city. Thus a nice shrine without investing in missionaries.
RE: Chop/whip a settler. Before the granary to try and grab one more city site. Maybe 1S of the cows to block mansa from plopping a city there. A third city can really help with troopage. Then The Holy War between the athiests. I would focus on taking Medina, Mecca and any cities between Mecca and Celtia. Or cities with Jumbos. This seals off the western pennisula. I wouldn't try it with a msall stack. Think 10-12 GW's and one axe one spearman for defending any cities you take. That is a minimum. He is protective and will have cultural defenses due to Stonehenge. Taking saladin out completely will either crash the economy by keeping the cities. Or open up more land for Mansa to settle if they are razed. Taking a few prime city locations will give Sis a strong core empire with a few nice cities to the west. He will have a strong land advantage as well as population due to charasmatic plus resources. Saladin will be reduced to a small blob that is just holding spaces for Sis. After getting philo from a bulb. Math/construction/HBR unlocks cats and jumbos. At this point he should have a strong production base to pump out units. As well as a stable shrine boosted economy to support a war. Wipe saladin off the map, shortly followed by Mansa. No Vassals. Kill Kill and Kill.