If one Moderator/Owner currently starts to moderate a message, are allDepends on what you mean by "moderate" - the first moderator to Edit a pending message gets a "claim" on it. That prevents action by email, but only warns other moderators who attempt to act by web (they can override the "claim" warning by acting on the message despite it).
That is not (currently) true of Rejecting a message (which opens an edit box for the rejection notice.
It is also not true of simply opening the message from the list to look at it and maybe choose a button.
If more than one Moderator/Owner currently is moderating at the sameExcept for the Edit action, whoever finishes first prevails.
That is, if a bunch of moderators open a message to look at it, and one approves it then the rest (if they try to take an action) get a warning that the message no longer exists.
If there currently is a 'lock' that prevents more than oneI think a similar "lock" could be used, but I'd want it to be distinct in both appearance (so that mods would know whether it is a moderator claim or a sender's deletion request) and because I would want slightly different behavior.
In particular I wouldn't want a moderator's claim to prevent a member from making a deletion request. That's because I sometimes leave a claimed message in the queue for hours (or a day) while I consider it or ask another moderator about it. If the member meanwhile decides to withdraw the message I'd like to know about that, and not have them prevented from making the request.