Ima,First let me start by saying CONGRATS any coin from pre Colombian contact is a truly rare find and one to be treasured!!! They do on occasion happen herein the U.S.,I know first hand.I'll get to that later.
On your coin,I am no expert but the first thing I thought of when I started reading your post (I read every word) was Sir Francis Drake.One of the first explorers to your area and a known pirate.You've referenced him giving chase to a Chinese junk.He was,according to your research,within a hundred years time frame of the mintage date of your coin.You said your coin was found on the coast in the convergence of the tide and a fresh water stream.Fresh water was something constantly sought after by any vessel.If this is a beach it could have been used as a careening spot to heave down and clean the fouling from the hull,repair leaks and take on water and whatever provisions could be found or traded for.Its possible that someone may have been washing clothes and lost it a little further upstream and it washed down to the spot it settled in.Or the natives may have taken it in trade for supplies and then lost it,of course this is all just speculation and the wanderings of an imaginative mind but a possible scenario.
As for the coin itself,I am definitely not an expert on Chinese coinage,but I am somewhat of an expert on coin wear as to having been in the earth and surf.Coins of bronze or copper,as most know,don't hold up very well in salt water.Yours being in more brackish water appears the have survived better than most.
The rim edge is worn off,the coin according to you is thin.A coin with a center hole would also wear the outside edge of the center hole just as it does the outside edge of the coin.This could explain the larger center hole of your coin and not being able to find a perfect match.Is the coin holed or nail marked?Are you certain of either?Could have just been someones charm long ago on a necklace.
As for pre contact coins being found in the U.S., I know first hand because my dad found a C.87 A.D. roman bronze here on the coast of Maine! It has been authenticated.Who knows how it got there but its quite possibly the oldest coin ever found on U.S. soil.I have picked up what appears to be a button which is about the size and has the same beaded rim design of a widows mite an (early roman coin) and has the faint outline of a bust of a man in a helmet similar to what the roman soldiers used to wear.Also many chunks of natural chalk is found in the area.Most of the worlds natural chalk is found in England and France and the Romans were established in that region in that time frame.All points to the Romans being here, perhaps a ship blown off course long ago.The Vikings were supposed to have been here around 700 AD but that coin would have been ancient even then.The Phoenicians are believed to have had an ancient trade route along the New England coast,evidence of which has been found in carved in stone in Phoenician script.My dad,up until 2007, had never been out of the country and didn't have an eBay account and I know for a fact that he found it here,without question!
Well,once again congrats and WTG!!! Although it's not extremely valuable I know its priceless to you!!! HH, Bootstrap
PS. Didn't mean to hijack your post
just trying to give a little extra insight
and let people know that finds like that
ARE out there!