Most likely since tickets can be discussed in detail on the wiki amongst agents, and therefore a protective silence on the entire wiki has been put in place.

For the other questions (such as how to remove another user as an agent), OTRS admins are not really admins as in the normal wiki-way, but more like "rulers of OTRS". Their word is law (kinda). So, to change any of this, they (or WMF staffers) would need to do it.

-- Jonatan