You asked for the opinion. I gave it. It seems that you found three other questionable forgeries to back up your assumption that your questionable signature is real. If that is what you are looking for, then do NOT contact a Hemingway expert. You most likely will not be happy with their opinion. Of course, you can still ignore it and use the three "signatures" that you saw on the internet to trump the expert opinion. Like I said, most Hemingways on the market ARE forgeries. Using them to validate your signature is silly. I found these documents that I posted from major libraries and other trusted sources. They are LEGIT. I pointed out the MAJOR flaws with your signature compared with AUTHENTIC examples.
mjp is right. The only COA that has any value in this case is one from a well known Hemingway expert. Any book dealer can issue a COA. If you have a printer at home, you can issue a COA guaranteed by tmmasarri. I'm just not sure that means anything. Still, some ebay buyers think that a COA is the word of god, so maybe you can just make one up and sell it that way. It will carry as much weight as any other non-ABAA bookseller.
As far as 3rd hand provenance, mjp is also right. I would never buy a "a friend of a friend got is signed" as these stories are almost always bullshit. You can find tons of books signed by all kinds of famous people to "Bob" and the seller insists that "Bob" was their dad and that he knew the famous person back when. Of course, it is 100% unprovable and almost always made up.
A letter of provenance with your book would help, but Willard would have to be proven to be an associate of Hemingway. Otherwise, it is just some anonymous guy's story.
And you still have a huge problem with that "s" and the fact that the writing does not slant down to the right...
p.s. I don;t know any Hemingway experts, but maybe contact Butterfields, PBA, Sotheby's. Still, I would not tell them the Willard story, unless you want them to refuse to look at the book outright.