Yes, Amanda Tapping is Carter. They also even promote her - she's Major Carter for the first season, Colonel for the bulk of the show and promoted again for the last season or two. It's nice that they at least pretend like the higher ups really do notice when you save the universe :)

Having watched B5 when I could binge on the show, Byron actually wasn't that bad when done in that way, and was definitely worth having the fifth season (and Byron was only in the first third of the S5 episodes, and even then he was only the focus every other, IIRC).