the definite answer is... NO.
this may be a large "duh", but i had to see for myself.
even though the bosses were dead, trash had respawned. after a few attempts of running the gauntlet and getting mercilessly killed over and over, i managed to get past all the respawned trash up to mandokir's area.
i was even able to get into his room far enough to see the tied up lashtail hatchling and target it. however, it did NOT have an exclamation over its head or a quest for me to turn in, despite bloodlord mandokir being long dead in that particular dungeon ID. i have completed the lower level lashtail quest chain, have not completed the level 85 quest yet, so i'm still entirely eligible to receive the pet. the main thing is that i was not present for the boss fight or kill.
so my conclusion is that in order for players who have completed the pre-requisite quest line to be eligible for the pet, said players must be present for the bloodlord mandokir fight and kill. only then will the baby raptor quest be unlocked and you can obtain the pet.
or if someone else wants to take it step further, perhaps you just need to find an instance ID with a much fresher mandokir kill. like, within a few minutes of his death? the zone ID that i entered into had been cleared for many, many hours already. i'm not sure, and i highly doubt it, but perhaps time plays a part in whether or not the lashtail will offer the final storyline quest to you?
well, in any case, the trash alone is enough to deter me from attempting to go back in and "solo" the instance for the pet. gone are the days where a max level player could stealth in and run effortlessly around ZG to explore and do various fun/amusing things. :P it was quite interesting figuring out how to navigate amongst level 85 elites, though. haha.
if there are players who want to test out other theories or possibilities, feel free to do so at your own risk lol. ZG is not a friendly place! if someone does manage to figure out how to acquire this pet "solo", i'd love to hear how it's done. i won't be solo'ing ZG any time soon though.
now that my curiosity has been settled, i will have to wait for my instance lockouts to reset and find an actual ZG group to clear bloodlord mandokir for my shot at saving quintessence'aka!