There was a debate about a specific situation on twitter the other day, which is a variation on your point 5. What if a writer has sent you a query, hasn’t heard anything back from you (30 days haven’t passed), but has received an offer of representation from another agent. Would you want that writer to let you know about the other offer, even if you haven’t gotten to the query yet?
Interesting question. The answer depends on whether the query writer has decided to accept the offer.
If the author has decided to accept, and has only queried me (not sent a full) then no. If I request a full, then the author can reply that s/he has accepted an offer. If I send a rejection, the author can smugly smile and think "you think you're so smart Shark, ha on you."
However, IF the author is uncertain about accepting then yes. I can give the querier a yes or no pretty quickly at the query stage so if I am interested in reading the full, and there's enough time to do it, it's worth knowing about.
Now, I can actually hear all your devilish little minds whirring away here. You're thinking "how about I just email Snookums and tell her I have an offer! That way she'll read my query fast, and maybe request the full since she knows someone else wants it!!"
This is an ill-advised path. For starters, I'm more likely than not to pass on a project even if I like it a lot if I know you've got an agent on the hook. Second, I'm not swayed by someone telling me "another agent" likes this unless I know who it is. And honest to Godiva, you don't want to start telling me slithery Barbara Poelle likes something if she doesn't. She'll carve you up for breakfast sausage and feed you to the shark (yum!)
Does this answer the question?