As a principle, I don't see any reason to pay extra for a seat I already paid for at time of ticketing.
But you're completely wrong there: it did not.
My main question is...if there are no WT+ seats to allocate to me at T minus 24 up until door closing time, how great a risk am I at of getting IDB'd versus downgraded or upgraded?
You are extremely unlikely to be IDBd.