Well, the way I see it, it was a near-perfect ambush, withholding the knowledge of what exactly was in the drink until the last moment (and thus corresponding to lack of OOC "prescience") - this is how PvP is ought to be done, preferably with secret pms to GM and all that.
Why almost? Well, because of not waiting for the other player to take the bait while posting merry "give a drink" action; the "hold and cut throat" had to follow up afterwards, in the next turn. However, I understand the reasons here, and thus don't criticise the non-perfect execution of the plan too much.
To Abigail's side? Well, since (theoretically) there should not be any IC knowledge of treachery, the survival has to depend not on actions, but on side details and passive abilities. For example, I find it reasonably likely that she could get some illusions from just sniffing the vapours, and thus have IC reason to become suspicious of Flint - remember, Xeno-spit comes with a dropper for a reason; it's 4 drops that put you straight into "unconscious" territory, I think? (note that, however, should she accept the offering and
drink - chug, gulp down, etc. - I'd say thorough unconsciousness is the least that would follow). Furthermore, I believe that it is exactly a time when Intuition skill is ought to be rolled for a player character about-to-be-betrayed: it fits the original skill description (that with grenades unexpectedly coming into latrines, and so on) almost perfectly, after all. Finally, if the recipient of ambush had a neuro-reading amp, I'd also offer a chance of "reading" the minds of would-be ambushers at the last possible moment to avoid being caught flat-footed and defenseless. However, a proper ambush - trap, assassination attempt, etc. - ought to be lethal and very hard to dodge, because it's its very point.
As for "split time" - I say, put the mission start on hold and let them deal out with the situation, possibly in an expedited way (i.e. "no one except combatants and joining participants post actions" and make several small focused updates twice in more rapid succession (2x update rate, for example)). There is now a long-standing tradition of f*cking up missions even before the deployment, and some of the conflicts have become quite iconic and legendary, so I don't believe we should interfere with another contender entry for Hall of (Shameful) Fame of HMRC/ARM.
Anyway, to both main participants of the conflict: good luck!
However the fight ends, it would certainly provide worthy RP-material. And that is what matters in the long run.