There are all
sorts of command line tools for OSX. Probably the best way to get them is through
Homebrew (ffmpeg is certainly there).
Yes, you can use external encoders with the Mac version of MC just like you can with Windows. You have to get and know how to use the command line tools, though.
The Mac version of MC does not have the internal filesystem browser, though there are a bunch of options. As Brian pointed out, the RMCF Tool is probably your best bet. But you could also let Auto-Import's Fix Broken Links feature fix them if you really want to move things outside of MC.
The other way I know of, is to use the Tagging Pane. From Tagging, you can edit the file name and MC will change it in it's database AND on disk. You can use expressions here too, which is convenient.
If you change the [Filename] tag, MC moves the file. This is essentially what RMCF
does in Rename mode. So, any way you can change the Filename tag, you can move files.
RMCF, of course, also allows you to use expressions. I'm sure you knew this, but others might get the wrong impression from your post, so I wanted to clarify.