The Emacs michelson-mode relies on being able to call Tezos client. How can you use the Tezos sandboxed client for it?
You can open emacs in the same terminal as you ran:
eval `./src/bin_client/tezos-init-sandboxed-client.sh 1`
When running this script, a path corresponding to the sandbox tezos-client (it looks like
/tmp/tezos-tmp-client.XXXXXXX/bin) is added to the PATH env variable.
Alternatively, you may want to use the following command (with the correct path) in another terminal:
PATH=/tmp/tezos-tmp-client.XXXXXX/bin:$PATH emacs file.tz
Also, you can have a look at https://tezos.gitlab.io/master/user/sandbox.html for more information about the sandbox mode.