Welcome to the forum Jack,
Edster is the forum member who has done extensive research on the subject, and you can find his threads with a simple search, as well as his papers here, under Africa:
http://vikingsword.com/ethsword/
He studied the manufacturing process of kaskaras in Kassala in the 1980s and therefore has a better perspective on dating these swords than pretty much anyone else here. I am sure that he will chime in when he sees the thread.
Looking at the pictures I agree with your assessment. The blade with its three fullers follows a popular pattern of trade blades manufactured in Europe and exported to Africa in the 19th century, but European made blades from that period tend to have straight and even fullers, while these are rougher, suggesting local blacksmith work. The crossguard also looks like it is a form, associated with the 20th century as a simplified version of earlier guards that were more complex to forge out of 6 different pieces. Taking all this into account, it becomes really hard to place this sword in the late 19th century.
That being said, I would not be totally disappointed with it, because it is a good representation of the form as used during the Mahdiyya. Kaskaras with old imported blades and high quality fittings are increasingly hard to find, and this can serve as a placeholder until you source one for your collection. If it is any consolation, it helps to keep in mind that the kaskara remained in use as a male attribute in the Sudan until modern times, so even a 20th century sword would still have been carried and used within the original cultural context, even if not as a primary weapon.
Sincerely,
Teodor