This is a question that has intrigued me for a while now. My reading suggests many dates from the early 300's right up to 1066, all of which have valid points and flaws, so its difficult to pin down a real date.
To just list a few and the motivation behind their nomination:
Does anyone have any other suggestions or opinions on when exactly the Medieval Period began?
There is no hard and fast answer. Periodization is always more than a bit arbitrary, and mostly based on convention. In reality, late antiquity blends into the early middle ages almost seamlessly, and the exact date of demarcation is a matter of personal preference.
However, I think most historians would agree that your first two dates are solidly within the area of late antiquity, while your last two are solidly medieval. Proposed starting points for the middle ages are usually somewhere between the collapse of western Rome and Justinian's war in Italy (which did more than anything to wreck the remnants of Roman Italy), and maybe the beginning of the Islamic conquests. So, 450-630.