You can not re-attach a process id. You need to reattach the corresponding tmux session.

So do tmux ls. Pick whatever session you want to re-attach. Then do tmux attach -d -t <session id> to re-attach it to a new tmux instance and release it from the old one.

Nice idea. I'd create a wrapper, say /bin/my-screen that would look like this:

#!/usr/bin/env sh

screen -d -RR

Make it executable and add it to /etc/shells:

echo /bin/my-screen | sudo tee -a /etc/shells

Make it the default shell:

chsh --shell /bin/my-screen

Notice that some terminal emulators such as xterm do not run shell defined in /etc/passwd by default but they check $SHELL variable first.

You can try adding this to your ~/.screenrc:

screen -X screen -d -RR

Notice however that there is a potential problem with that solution - you won't be able to start screen at all if there are no existing sessions screen could re-attach. That means that if you set your screen as your default shell you won't be able to log into your system.