My experimentation suggests that your inbox is kind of read-only. The only way to reply or vote is to go back to the top level thread.

I have some concerns over that. Unless there is something I'm missing, there is no reason for your inbox to be anything other than a filtered view of the communities where you've commented. As such, you should be able to do anything in your inbox that you can do anywhere else.

I could be completely out to lunch, though. My programming career ended over a decade ago and I have not done anything that resembles a formal analysis.