Continuing discussion from here.
I’m also in the “Reply button should be at the bottom” camp. I also think there’s no need to show two Reply buttons. Here are some mockups:
Right-align all the things:
Just dock the Reply button
Transition from vertical to bottom reply button
Either the vertical button could just disappear at the end, while the bottom reply button is always there. OR, we could get fancy, and make it so that when the vertical reply button reaches the bottom, we swoosh (aka animate) it over to the left.