The bit about Nero having Borg tech makes no sense, because the movie timeline does not allow for him to obtain any between the destruction of Romulus and the trip to the past, and I can't see him having any in his original career as a humble miner...
Because he was originally supposed to use ALL of it. Jellyfish, in that case, was basically a giant manned torpedo with the red matter as its warhead.
It's very difficult to see how a mission that originally called for use of all the red matter would suddenly be satisfactorily (if somewhat belatedly) concluded by only using a tiny droplet of it!
But if Spock really could only stop the supernova (from existing, or from destroying Romulus, or whatever) by being there in time, it would make sense for him to tap his source of supernova-nullifying destruction for a single droplet that allows him to travel to the past, and then
use the rest of it.
This only requires us to assume that Spock blatantly lied to Kirk about key facts during the mind meld - and since Spock is an expert melder and Kirk has never experienced the thing, it may well be that Spock could pull Kirk's leg a mile or a dozen. And if Spock lies about the going-to-past thing, then the whole meld might be full of lies, and there could indeed be room there for all the otherwise impossible things described in the Countdown
Why would Spock lie? He says he only used a droplet and achieved something (supposedly the salvation of the galaxy), then began a trip back to Vulcan, but was intercepted by Nero and sent to the past. This must be a lie if his intent was to travel a few hours to the past to negate the supernova with a large amount of red matter. OTOH, if this really was his intent (and a very plausible one at that), we must assume that he never got the chance to complete it, because his supply of red matter survives. So, in his original timeline, not only Romulus but the entire galaxy was destroyed...
In general terms, Spock already 'fesses up on the ability to time-travel with red matter black holes, so his motivation for lying can't be keeping this secret from Kirk. His story also fails to hide his failure, so that can't be it, either. Plus I don't see Spock trying to hide the fact that he has failed.
Any ideas on how Spock's story could or should be a lie?
...And how this would affect the weaponry of the Vengeance
? I guess Starfleet would insist on getting the specs of red matter from Spock Prime for perfectly legitimate reasons, and then apply these in illegitimate ways. More generally, Spock's ethics in lying to Kirk would reflect on his ethics in teaching the 23rd century Starfleet how to build futuro-weapons.