I think Ed gave a pretty good stab at this. I'll add a few points.
The relative political stability of Ethiopia combined with a habit of outsourcing blade manufacture in the (caveat) 19th century, means that for a certain period the shifting sands, shift, well a bit less. Otherwise you can find kaskara even crossing over, include, if memory serves correct in church iconography/wall painting.
Kaskara use extended into Bornu, so it certainly went west, Ethiopian weaponry I think stayed a bit more bottled up, partly due to cultural/religious differences and partly just due to geographical factors. Natural barriers and the type of trade going through Ethiopia from the Sahel.
The date of popular kaskara use I also have no idea about, but I have often wondered if the Mahdist period ignited some interest in the type due to association with the sword of Mohammed and religious connections with swords in Islam.
As Ed noted Kassala was relatively late to the party in terms of being an arms center. Omdurman was noted for mail product and other military gear during the Mahdi years, however earlier than that less is known at least to me, about Sudan specifically.
What I do have to go on are a few period sources like Barth reporting on the Kano re-export business. I still have a problem understanding why less of the same trade blades in kaskara turn up in takouba and also why kaskara hilts don't seem to be know at all in Hausa areas. Sometimes you get cross over - I'm attaching a Peter Kull bladed takouba with re profiled tip that sold in Germany a little while back (unfortunately not to me). THis is the exact same blade as found in kaskara, just profiled as a takouba. However these are few and far between in my experience. However the vast majority of takouba and kaskara blades don't really match up, particularly the wide fullered kaskara - making me question if the form was carried into Sudan from the west. I would still subscribe to the Mamluk influence theory and export of weaponry from Cairo. Even maille from Cairo was showing up in Egypt so any weapons trade certainly went both ways.
In short, you don't really find a spot where the sands don't shift a bit in the Sahel. You have two wildly different hilt types in the kaskara and takouba, that seem to have shared some common sources for blades, you have long and well used trade routes to propagate forms outside of their source zones and you have similar styles of warfare and large empires/kingdoms bumping into each other. In short it's a highly enjoyable mess.
Best,
Iain