I don't know that there is a "right" answer to these, only intelligent thoughts and reasoning (plus some research into historicity).
"Could" other shapes be used to measure area? Sure; why not? But how useful would those other shapes be, especially considering how one customarily (that is, historically) measures the simplest shapes? Did "mathematicians" even do the choosing? (I rather doubt it. Mensuration was fairly well established back in ancient Egypt, when "math", such as it was, was still a religious pursuit. The profession of mathematician didn't arise until millenia later, at least in any recognizable form.)
As for choosing grids, isn't that just a matter of choice? Granted, some choices are probably better than others,
for a given application, but there is generally more than just one useful choice.
In any case, this question is asking for
your thoughts, so... do some thinking!
Eliz.