Sentinel,
The most accurate account of what happened to the "Fish Map" after Frank Fish died, can be found under the topic of "The Fish Map" on the LDM Forum. The post by Greg Davis on Apr. 22, 2004, is as good as it gets.
As to how Frank Fish actually came by the map, that's always going to be up for debate.
It seems to me, that you have to go on the assumption that the map is the real deal. If you assume otherwise, without any material evidence, you gain nothing positive. The same reasoning can be used on most treasure stories.
All of the above is just opinion.
Good luck and good hunting,
Joe