But are the shuros and “hamelech” a minhag or hidur. What is the common one sold amongst the ashkenazim
I don't know about Megilla specifically, but Tosefos in Sotah discusses opinions that Shuros are mandatory in all Kisvei Kodesh, so I would think it's more of a chumra than a hidur.
What do you mean by "Shuros"?
I understood it to mean the layout of the page such as which words on which line and whether the 10 sons of Haman are on the top of the page or the bottom, or on their own page etc.
If you meant Sirtut. I think that all Megillos today have Sirtut but I may be wrong.