I would like to ask that OP please x-post this question to /r/linguistics or even /r/asklinguistics, as this is fundamentally a question about historical reconstruction of languages. Also, we don't have a proper Arabic philologist, and they do.

Follow up question, if a specific dialect could be understood (ex: Coptic and ancient Egyptian), which one and why?

Linguists term modern-day Arabic Modern Standard Arabic ("MSA"), and Classical Arabic being the term used to refer to the earlier form the of the language which includes the time of Muhammed. It's worth noting that present-day speakers commonly refer to both as al-Fuṣḥá; this is due to the fact that MSA is a simplified and modernized version of Classical Arabic.

Now can an MSA-speaker understand Classical Arabic? Most likely yes, the difference between MSA and Classical Arabic does not make the latter incomprehensible to speakers of the former. The Arabic Language by Kees Versteegh can be a useful source for this topic.