Elite097What I was getting at is that the author is using two cases (English and Aymara) to support the idea that the language one speaks affects how one visualizes time. It's certainly possible that speakers of other languages move differently and visualize time differently. However, this wouldn't hurt the argument at all, so there's no assumption being made about how many different ways people can move when speaking about time. The author isn't saying
"Clearly, everyone moves either backward or forward when talking about the past or the future." They are just using these examples to talk about visualization. If speakers of some other language see the future as up and the past as down, and if they look in those directions when speaking, that doesn't harm the argument.
As for your second question, I think you're adding things that we don't really know. That's exactly what an assumption is! First, we don't know that you have to visualize in order to move, especially when we are talking about small motions people make when speaking. If I lean back or forward when I am talking to you, I may not even realize I am doing that, let alone build a visual model of that action in my head. Second, the argument and answer choice D are not talking about how we visualize our actions; they are talking about how we visualize
time. We have no idea if our motions when speaking are connected to our mental visualization of time in particular. That's the assumption the author is making. Maybe some other cause is influencing those forward and backward motions the author is reporting on.