Check out Addison's Gun Shop located on 441 between Kissimmee and Davenport Florida.   He has had a pretty good selection in the past

This helmet doesn't need to have belonged to a Marine. It could have belonged to any number of kids who lived in base housing at Quantico who wanted to play army in the dirt. Lots of kids, lots of used military gear, and everyone (at least back in the day) loved playing army in the dirt (we did, all the time, with WW2 issue helmets...) 

Again, I had stated that this could have easily been an off-base thing used by a Marine stationed at Quantico in the 1960s for use as a riding helmet or even a piece painted up for fun. But there is zero proof or evidence this is a WW2 Marine Raider helmet and others have said as much. 

You can use that site with different years in place of what I sent you. Look for that guys NOK

Well he’s dead, so that’s a shame, back to square one, seams like ghat guy might hav been in country while he was, I also searched the sight you PM’d me and there’s nothing. I tried searching Missouri and Kansas death for OK residents and I checked for Muscogee specifically to. Still nothing. There’s plenty of Harjo’s, but no our guy.

... must admit, in fact, the converted Sharps carbine has so much appeal to me, more than a Trapdoor-style one, and my original plan was of a full-size character in a timeframe correctly suited for the gun. This however meant (speaking of a regular cavalryman) something like from about 1869 - 70 to mid-1870s so, no way in my opinion of finding correct items in terms of clothing and equipments for such a critical era. Thus my choice of stepping forward to around 1890 - 91, still fascinating stuff but decidedly less hard to find (and especially to get, from here in Europe). I'll post some items if you like to see them.   

Start digging!!!!

I did search up the harry simspon guy on tinder a grave, and guess what came up first? A Native American looking man who was with the 11th Cav. In Vietnam. Might have known him.

Speaking of Line Officers, does that mean the same thing in the Navy as it did/does in the Army? Officers assigned to shipboard duties as opposed to rear areas?

There's no telling. This could date from the 1930s through the 1980s. Maybe even the 90s, but the patina looks older than that and it's a "real" bullion star rather than the "fake" tinsle type used currently. That said, vintage star sets are still available and people are known to use them, even today. The shape of the star didn't change and they were made by many makers, so small variances are known.