As a native speaker of Sicilian, a Romance language with a lot of Greek in it from two separate periods of Greek influence on the development of the language, I'm very skeptical about how much a Romance language will take from Greek phonologically.
In terms of vocabulary, Sicilian for example takes a lot from Greek, but I can't see Greek say, turning the shift from velars before front vowels to post-alveolars/palatals into velar *fricatives instead. I can see it preventing the change all together though, since Standard Greek seems to have retained velar stops before front and mid vowels. The aforementioned shift is absent in Sardinian and Dalmatian...
For a surviving, vibrant African Romance language (there were probably a lot of African Romance languages though, considering the ground to cover), I think a POD after the collapse of the Roman Empire that prevents the birth of Islam. The reason that Arabic has spread so vastly over the Muslim world is because the Qur'an for many centuries was not translated out of the original Arabic so that meaning was not lost (unlike the Bible, lol). If people wanted to be Muslim, which is a faith that arguably takes up a considerable amount of your daily life when compared to Medieval Christianity (praying five times a day), then you had to speak Arabic. I have nothing against Muslims, but their religion has been responsible for a significant amount of language death and suppression because of this. If you prevent a religion like this from arising, then there's no reason that African Romance wouldn't survive in my opinion.
Panonian Romance was also still around probably until the Magyars settled in Europe proper. You could prevent that from happening as well. It's probably a little more plausible than preventing a Saxon invasion of England, considering the differing motivations.
EDIT: I don't know about a Romance language in China, but there were a group of Indo-European languages spoken in Western China called Tocharian languages. Perhaps you could prevent their extinction somehow? I'm really not that familiar with the people that spoke them or why they went extinct though. Sogdian languages also represent a very interesting branch of the Iranian family that is only represented by Yaghnobi today.