This comes from someone who has never done anything more than vet FPs and solo content on sentinels/marauders.
As for the specs:
Watchman:
15: Juyo Melt is pretty good. Melting Center looks good, but the 4x damage seems to not trigger or I couldn't tell. The 10 stacks of Centering helps trigger Zen quicker and more often. Sounds good in group content for some more offheals.
20: Transcendence should be a given ability. More reasoning below.
30: Not sure how I feel about Force Lash. Flaming Wave is great. It adds more aoe, when you had to wait for Sweep to go off cd and reapply the dots before using Sweep again.
35: Force Clarity should be a given, since the gear set for it works better on Watchman. Burning Center should be something more, but like Melting Center, it does help use Zen more. Burning Zen sounds good, but wouldn't shine if Force Clarity is an option and not a given ability.
50/60: choices are fine. Need more movement in fights? Double Leap. More defenses? Take that one. For 60, Cyclone isn't as useful as it is for Combat. Jedi Adjudicator and Zealous Ward are good for soloing.
70: I would swap Guarded with Stoic, personally. Stoic is far more useful for solo sentinels/marauders than Guarded is (I have not used Guarded, but players who do use it would take it)
80: The choices are not great as the other levels. If Stoic stays where it is now, then that is the only useful option for solo players.
Combat:
15: Puncture sounds great, but it would require positioning a lot more and with mobs liking to surround you, it would be a pain to make the best out of it. But in mob heavy fights, it would work better. The cd reset doesn't look appealing. If it did something more than just an armor debuff. For Driving Lance, it would work better if you had a way to track your movement speed.
20: Transcendence should be a given ability, since Combat/Carnage always lack as much defensives as the other specs.
30: I like the fact that using Blade Rush boosts your DR. Can't test Rush Down (obviously), but it has the issue as Driving Lance, but with the added issue of clarification. How fast you are moving when using it (like you need to be circling the target for the damage boost) or is it like Driving Lance? Valor Blade sounds good. But Valorous Call is not good enough to warrant a cd reduction per Blade Rush use. If Valorous Call did more than just max out Centering, it would be better.
35: I am sure others have said Force Clarity should be baseline/a given ability. I am on the opposite side. It should stay a choice for Combat, even if its just because of the gear set for Force Clarity. It is very weird on sentinels/marauders. Swiftness sounds good, but the duration should be increased by at least a second (miss one or two crits and it resets, not to mention how useful the speed boost is for Lance and Blade Rush), but not sure how balanced that would be.
50: Same as Watchman.
60: Trailblazer is good. A solo Combat player doesn't lose out on any DR boosts or defensives. But if the free swap stays, they can take Zealous Ward if needed.
70: Same as Watchman.
80: See level 70.