Giving the problems I see from the least important to the most
1) You have two premises numbered '2'
2) I am not sure how you derived the conclusion that the future cannot change, although if you include the possiblity that the future does not exist then I suppsoe it is meaningless to say that it could be changed. (Note that "the future" typically refers to the future state of the universe).
3) It is not clear that tiem does not have a lifespan, it might be said to have an infinite lifespan (if time is infinite) or a finite lifespan if it is finite (since it cannot exist before or after itself)
4) It would be best to show your working. It is hard to tell if your derivations are valid when they are not provided. Equally your definitions need to be clear - consider the point above about "lifespan", for a start.
5) To show that a timeless god can exist you need to show that the definition of "god" is compatible with timeless existence. For instance if a god is required to be capable of acting it could be argued that that entails change and thus is incompatible with timeless existence. It would certainly be odd to talk of a god who was incapable of doing anything..
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