The First Space Lord was a member of the Manticoran Admiralty and the senior uniformed officer of the Royal Manticoran Navy.
Mark Sarnow would certainly be capable and would presumably be out of the area of the naval decapitation strike. The other comparable remote area commander, Augustus Khumalo, could be considered too. He's shown plenty ability to back up subordinates making sweeping decisions while retaining authority, and the Talbott and Silesia commands will have acquainted both these officers better than plausible alternatives with high-level policy decision-making.
The Second Space Lord was one of the Lords of Admiralty who controlled the Royal Manticoran Navy, responsible for handling operational and tactical planning. He or she was also the head of the Office of Naval Intelligence and the Bureau of Planning.
I'd be so very amused to see Anton Zilwicki tapped for this, but it's unlikely to happen. I don't think we see senior subordinates of the Second Space Lord often, and they'd be likely candidates and plausibly spared that decapitation event.
The Third Space Lord was a member of the Lords of Admiralty that controlled the Royal Manticoran Navy, in charge of the Bureau of Ships.
No idea. After Oyster Bay, I imagine many of the most senior and experienced candidates are already dead, so they may have to dig down for one here. I could see pulling in someone from the RHN or Beowulf's SDF here more easily than for a lot of other cases, assuming Grayson is equally hard-up for senior officers with relevant experience who could serve on loan. Shannon Foraker's Bolthole experience would serve the GA in good stead here, and it'd likely put her in a good position still to use her idea-hamster superpowers for Good.
The Fourth Space Lord was a member of the Lords of Admiralty that controlled the Royal Manticoran Navy, in charge of the Bureau of Weapons.
As such, he or she was responsible for the research and development of new systems for the Navy. (HH9)
Sonja Hemphill, slam-dunk, assuming she isn't caught in the strike too.
The Fifth Space Lord was a member of the Lords of Admiralty who controlled the Royal Manticoran Navy. The Fifth Space Lord was responsible for the Bureau of Personnel (BuPers for short), which handled recruitment, training, and personnel assignments for the Navy.
I would so much hate to be the person who would have to fill Admiral Cortez' shoes. No idea.
The Sixth Space Lord was a member of the Lords of he Admiralty who controlled the Royal Manticoran Navy. The Sixth Space Lord was responsible for the Bureau of Training (BuTrain for short), which handled the training and education of naval personnel in accordance with the Third Lord of Admiralty.
For narrative reasons, and maybe even seniority by now, we're even less likely to see her here than in the First or Second Lord's seat, but her academy experience and skill, and what happens to the efficiency of officers serving under her make Admiral Alexander-Harrington an awesome candidate here.
The Seventh Space Lord was the seventh most senior uniformed officer of the Royal Manticoran Navy, holding the offices of Surgeon General of the Star Kingdom of Manticore and head of the Bureau of Medicine.
Seconding (at least) Fritz Montoya for this one.