In the US there is a trend to joint commands structure where where the commander of an instalation is one branch of service but units and personel under him or her could be from many branches.

its why you see air force bomb detection K9 units and  forward air controllers embedded in army and other units in war zones now.

With as small as the RM are in setting I think that you will see a JAG corp made up of all branches of uniform service from the Planet Clarion. Those would be Royal Guard and Royal Marine as canon and anything else me might suppose exist on Clarion- the Royal Sea Guard? it doesn't seem that Clarion would require a wet navy per se but something like the US Coast Guard? It is a fairly wet world so its probably not a stretch to include the existance of a Coast Guard.

That said- the emblem should probably default to the royal aspect- a lion? as the monarchy would supercede all branches. probably have a scale- as in the scales of justice? and maybe a sword crossed with a quill? the sword being emblematic of military and or the sword of state and the quill a recognition of the more or less legal writing part of the job.

I do like the leaves used in the above symbols but not sure why they were used- the stylized version in the star trek one is particularly nice IMO.

Yeah, it's probably best to avoid overt American and earthly royal stuff for the symbolism. Otherwise it might look too derived and not 'scifi' enough IMHO. Something that reflects the marine's history in terms of Star Frontiers would work better, I'd say.

If we default to the canon monarchy we're pretty much going with a lion it would seem which evoks the British monarch upon which the Clarion monarchy is based.

what sci fi elements would keep this from being derivative?

Not sure if its possible to keep from being derivative at least with the planet clarion

So i'm for a JAG that will have uniformed personal from the Royal Guard, Royal Marines and Royal Sea Guard just because a I dont see either the Royal Guard or the Royal Marines being all that large so it would make sense for the JAG to be an joint/intra service department with the tradition that a defendant gets a defense lawyer from his service.

dominant element of the symbol would represent the monarchy- possibly a lion

other possible elements: balanced scales, laurel leaves, quill and sword

Possible sci-fi elements? classic laser rifle from SF illustrations?

A character in the JAG would need 1 level of law skill minimum and would function as an investigator although if adventures are patterned after the TV show there will be a lot of talking to NPCs and maybe skill checks with computer data bases.

I suppose you could set things up where the accused is stone walling/lying and referee can pretend to roll INT check and tell the player "You believe hes being untruthfull" If player uncovers and confronts the NPC with evidence of his lie then the NPC tells detail X.

Adventure would consist of 3-4 NPC interactions for information- role play and ability checks.

Some skill checks.

Well crafted and it could at least include combat of some sort.

Friendly Fire with Densel Washington and Meg Rayan would be great inspiration for this as well as A Few Good Men.

Something like this could work.

However its hard to escape the fact that Earth Tropes were built into the
Clarion setting: Militia ships named Leo, Osprey Eagle and etc.