I'd argue he can - though I have to say, if we're talking work, SGA is probably not the best example to draw from to decide if Joe has acting ability or not, since I agree with you - I think he struggles to find that particular character, making Sheppard seem a little 'forced' at times.

I think Sheppard is an interesting character, definitely made by the choices Joe makes as the person playing him, and I think the way he's played it means something like this fic could easily slot into the canon-verse.

On the issue of the fic, I love the fact that you made Sheppard evil, and the fact that you tied Ba'al and the Goa'uld in. It was really well-written and believable.