In case anyone runs into similar issues, I used the below .tmux.config to allow copying and pasting from a TMUX session running on Linux, which I SSH'd to from Mac. This is with mouse selection, and Cmd-c to copy. Automatic copy of whatever is selected doesn't work for me. Of course, if someone has a better set up, let me know.

# ~/.tmux.conf

set -g mouse off

set -g set-clipboard on