most prostetics are slow to react and require a lot of brain/will power to work. considering this will be used for what i assume is fast paced gaming i doubt 5years is enough time. unless of course the prostetic is an extension of the nubbin left in place of the finger (still not 100% sure if op has some) so he's just have to move a different muscle to do it.
You could use A, S, D, and space. Move forwards with s, and backwards with space. You will probably have to rebind a few things to mouse 4 and 5 if this is an option; and the things that are usually on the number keys one row down. As previously suggested, an MMO mouse would probably be a good idea for games that support macros. Also an ISO keyboard might help in case you don't have one, since it has the additional button in that area.
Nvm
Well I think that with HOURS of practice you will adapt my friend. No one pushes A & D at the same time do they?
I only use three fingers to play a FPS (tf2)