Wife wants to talk one month after moving out...

[TrentC]

I'm not sure whether to keep the conversation topical, talk about what our path forward is (R talk), or simply follow her lead.

Follow her lead. Don't bring up the R unless she brings it up first.

Am I supposed to act totally happy, or tell her that I think about her constantly & miss her terribly?

How about in the middle? Just be upbeat and positive; don't come across as needy or clingy, as those are not attractive qualities.