Is a plausible hypothesis. As we all know viruses do not fosilize easily, the origin of the virus is unknown (apart that is linked with tropical forest canopies in Africa). But there are several traits that would suggest a plant patogen that jump later to animals, maybe via plant sucking arthropods, and stablish later in bigger animals eating this arthropods. I could be wrong, I don't remember when I read it, but I remember that the idea triggered a click, and makes a lot of sense to me. I'm not expert in viruses evolution in any case so take this with a pinch of salt.

Ebolavirus is a genus comprising at least five species that can create similar outbreaks. I don't know if the treatment protects from all of them (Probably not). Is a fast mutating kind of creatures also. I guess that each future outbreak will need to rebuild an specifical treatment. The good news is that we know how to rebuild it fast, there is a new awareness of the problem, and african people are fixing some cultural traditions that helped to spread the virus in the past.

There are also other two genuses closely relative to Ebolavirus: Marburgvirus (produces a similar haemorrhagic fever in humans), and the spanish surprise Lloviu cuevavirus (poorly known but assumed harmless). Treatment designed for Ebola probably will not protect from Marburg disease, but I'm speculating.

If you ever find a source for the virus for plant origin, I'd like to read it. (I'm not a biologist, but I'm curious. For example, I know about the relation with the Marburg disease.)