Mostly a SF role, honestly more likely to draw on specialist assistance from the SCE special operations unit (which I headcanon as the Pioneers), rather than the EF most of the time.
In a Prime setting... yes.
In this particular setting, no I don't think so. Under normal circumstances, the best Starfleet is going to have is a Redshirt. More the mainline Trek style of "Something is happening? Send the bridge crew over for some reason all the time."
Other than its existence being canon (and that's basically all we know about it), nothing really.
I kind of headcanon it has more of a specialist training cadre in terms of it's contributions to the Federation/Starfleet, perhaps similar to https://www.eliteukforces.info/royal-marines/mountain-leaders/ as it's regular deployments are largely going to be local matters as you say.
Yeah even in straight canon it's still a bit of an oddity. I would probably put it more in line with some sort of Federation-funded wet navy program member worlds can get in on. Probably much closer to a coast guard, with any actual command being local. The "Federation" is part is more of a civilian administration for funding/bureaucracy purposes.
.How "separate" it was pre-Khitomer is somewhat variable, though I generally have it at least one level below the Starfleet Commander-in-Chief/Chief of Staff
In canon, it may not even exist.
Realistically, if Starfleet Marines did exist in canon, they're just a department of Starfleet. Everything is a department of Starfleet.
Similar to above, I have no problem with the idea of a "National Guard" style strategic administration/reserve system, but certainly "combat arms" units (especially armor/artillery) shouldn't be a regular thing in Federation control outside of wartime IMO.
I would generally agree, which is why I initially had the idea of the Federation's military being more akin to NATO. In my initial idea here, the Federation doesn't have any military at all... it's member worlds have militaries. Starfleet "isn't a military", and the
If we're talking straight Prime canon... I actually think even Starfleet isn't actually in Federation control directly until after ST6. The main reason I say that is that it's clear the command structure changed between ST6 and DS9... in ST6, the CinC and Federation President are not the same, but in DS9, the Federation President became the CinC of Starfleet. I think that, in canon, it's actually kind of the opposite of what I was suggesting in my universe... Khitomer attaches Starfleet to Federation control?
Under the Accords, I generally see the reporting line being something like the https://en.wikipedia.org/wiki/Royal_Marines.
Yeah, I think that's better Post-Accords. I like the idea of it being a somewhat larger service Pre-Accords, being a branch of Starfleet but then Post-Accords, reorganizing to a department within Starfleet and a leaner organization. The Accords wouldn't have COMPLETELY eliminate any trace of military, just severely reduced the size and scope of available forces.