I don't know wether feedback is still relevant to current development, but off the top of my head I suggest the following:
a) all those you mentioned.
Also in no particular order: /leave or /part for leaving the channel, /help for an overview of the slash commands and/or additional info. There certainly are more, but that's what jumps my mind in this moment
b) I fell like it could be this: without the mouse hovering any entry of the slash-command preview display the explanation ending with a dot, followed by Syntax: ... which then unfolds into the full line (maybe with wordwrap) for the explanation including all relevant syntax.
e.g.: without mouse hover:
/loadtest users Add a specified number of random users to current team. Syntax: ...
with mouse hovering over the entry:
/loadtest users Add a specified number of random users to current team. Syntax: /loadtest users < Min Users > < Max Users >
Sorry, can't format it better right now.