I don't understand why the villains portrayal matters to the movie? Wasn't he the 'classic' Deadpool until they started altering him, or am I remebering that wrong? (Honest question)
And (another honest question) do you know for certain that it was a Fox exec and not an overzealous writer who wanted to bring his own personal touch to the X-Men universe who decided on Deadpool's portrayal?
Also, usually when an actor isn't pleased with how a movie turned out it's because 1. The movie didn't go over well, and the actor wants to distance himself from it so the Hollywood elites won't associate them with bad movies or 2. Because it wasn't edited to their liking, meaning the scenes and/or cuts of those scenes that the director chose weren't to the actors taste, but again that criticism is usually reserved for movies that did not go over with audiences. If you know for certain that their objections were because of executive pressure, then I stand to be corrected.
When Hugh Jackman and Ryan Reynolds got their scripts, they should have known exactly what the producers and writers wanted to do with each character. Scripts are more than just lines; they are detailed worded descriptions of the character. Everything from what the character says, where they are facing, and where they are moving.
It might not have been so easy for RR, but HJ threatening to walk off because of drastic changes to the script would have to make any exec think twice. So either the artists submitted themselves to the changes, or knew well in advance.
As for the actors signing contracts that allow them a more creative say in the movie, I don't know if it went through, but a girl I went to high school with and is now trying to become an actress, told me a while back, that the Actor's union (is it called the Actor's Guild, or is that just the awards show?) was pushing all studios to give such rights to actors.
Either way, the final product is the art. Once you start asking how it was made, you start to lose objectivity. Not saying not to question.
Edit: For clarity