In response to your fears about what was found by ettercap, I would say you really just need to adjust your router settings. I assume you have wireless turned on with no encryption or key set. As you have discovered this is a mistake. I assume someone was just using your free wifi and it is probably unlikely that anyone actually got on your Slack system. I would check the services you have turned on to make sure nothing is on that shouldn't be. If it is, turn it off. Check the accounts have secure passwords.

On Pam, I would point out that Slack does not use it and Patrick Volkerding is not likely to add it until some security concerns are corrected. There are some tutorials on adding it, actually if you add dropline gnome, it will install pam (or it used to, I have not used it in a few years.)