I have no formal handwriting analysis training and am certainly not disagreeing with your judgement that the signatures were entered by two different persons. The differences in the 'M's and the '8's are, indeed, very obvious. However, I believe it can also be stated that the two signatures perhaps have areas of "similarity" in their general style,letter slant,letter closure properties, etc. Thus, there at least exists the possibility that both signatures were entered by the same person.
What are the ramifications if the same person, presumably Grandma, did in fact sign in both box 18a (as Madelyn Payne Dunham) and box 19a (as Martin Blaine E-?-n). Would this mean that the document must be declared bogus? I don't think we could conclude this!
It's my general impression that virtually all the major players involved in this entire mess, including Grandma, were very adept at "gaming" the system. We have no way of knowing what really transpired 50 or so years ago when the Blaine COLB application document was submitted. I assume conditions in the government records offices at that time were rather disorganized. Hawaii had only been a state for a few years. Perhaps Grandma couldn't or didn't wish to involve another live person as "witness" in this Kenyan-birth, mixed-race matter. Maybe she chose to just forge the name given in box 19a and submit the document herself to the local registrar! Who could now truthfully say that this could not have happened?
Thus, even if the same person signed on 18a and 19a, I believe the document could still be authentic.
“It's my general impression that virtually all the major players involved in this entire mess, including Grandma, were very adept at “gaming” the system.”
What I like about this document is that if accepted on its face, it doesn't require any conspiracy on the part of Grandma. It says she is reporting a birth in Kenya.
My impression is that this form is not a take-home-and-fill-in form, but rather a form that would be filled in by the local registrar office clerk based on a visit to the local registrar office by the witnesses. The witnesses would give the information, the registrar would type it in in the approved format and language (that couldn't be known to a person who was not a registrar clerk) and then the witnesses would be asked to sign the form in front of the clerk, then the clerk would sign. All three signatures are on the same date, presumably at the same office visit.
The name “Blaine” appearing as the legible middle name of the 19a witness and also as the last name of the attorney who put this on the web raises the possibility of a family link between the two. This further reinforces the possibility that the 19a witness is a real person and not made up by Madelyn.
Professions can run in families and the 19a Blaine middle name witness could be an attorney as well, perhaps Madelyn's family attorney who might actually be the source of the document, but it could just be a friend.
The document, if genuine, was altered on its face with the footnote sometime after 1982, so if the 19a witness who signed the document was the person who gave it to Ohio attorney William E. Blaine, the witness would have had to have obtained a copy of the document subsequent to 1982. Perhaps Madelyn and her attorney or friend of 20 years at this point were involved in trying to clarify BHO II’s legal status at that time, for example to help him get a US passport.