Sorry for the delay @Clint_Dovholuk, and thanks for posting the video!
In terms of the Slack/Mattermost confusion - if you disable the 'chat integration slack enabled` setting in the admin panel, then the Slack tab should disappear, leaving just “Mattermost”.
There is a UX issue here on our side. The option labelled ‘(no categories)’ is what you need. But clearly that is an awful label name, and we need to get that fixed up.
In terms of the slash commands, were you able to give this a try?:
(i.e. omitting a category name completely)