I still wouldn't see him skipping all those ranks to go straight to Admiral. Honestly, being Hoods successor-in-training would make the most sense. Wasn't Osman a captain before she was promoted to Admiral and subsequent head of ONI?
Well it is stated that Margaret Parangosky and Terrance Hood is fond of Thomas Lasky... in the way of a future leader, much like Serin Osman. Plus Thomas Lasky easily demonstrated his loyalty to not only the UNSC but also Lord Terrance Hood.
I have a feeling that Thomas Lasky will be promoted to Admiral, from the recommendations of Terrance Hood and Serin Osman. This would occur after Operation: ATHENA's successful mission and the UNSC Infinity stopping the Covenant Remnant from acquiring all Forerunner technology. However Catherine Halsey will manage to escape with Jul 'Mdama who plans to finish the Arbiter once and for all...This would seem like the most logical reason as I can't see someone going straight from Captain to Admiral without doing something exceptionally important. Preventing Jul from acquiring all of that Forerunner technology would be exactly that. More so if they manage to regain both halves of the Janus Key as well.
That to me speaks a viable reason for a promotion...
- Diplomatic Peace with the Sangheili and Jiralhanae
- Reconnecting Communications with Vessels
- Investigation of a UNSC Traitor
- Janus Key
- Possibly Saving Catherine Halsey
Thomas Lasky was promoted to Captain of the UNSC Infinity, for a good six months, at least, prior to the events occurring in Halo: Escalation... 13 - 14 months post Halo: Escalation.
If a person shows merits of courage, leadership, loyalty, diligence, endurance... they are easily on the grounds for a promotion.
It should be noted that Terrence Hood is now 68 years of age and possibly paraplegic, post Operation: ATHENA, and so perhaps he has taken up a more passive role behind a desk, which would require a representative ambassador to perform diplomatic solutions with the Sangheili and possibly other client species?