I like that D_Odds, good points on Samwise, I would give Merry a level or two over Sam though and more later, but that is only due to the history for the game I have laid out.
Frodo was difficult to give a class to. He does not really fit in terms of D&D.
I like your take on trying to give Gandalf a value, but it is hard to measure his abilities in game terms. A rule of thumb of F10/MU40+ seems fear, but I can’t really back it up with the spells he used. Now he some sort of incredibly strong stealth magic to sneak into Dol Guldur. We know there was little he could not do with fire and he could throw up a Wizard lock that a Balrog could not Knock but I am not sure how to define most of his magic. The same is true of Saruman I think. It is hard to define them in game terms.
The Witchking to me is a power spectre with MU/Cleric abilities. So we are on the same page. I can’t see him being more than 20/20 though, so it would have to be a bigger gap then 7 levels for me.
Aragorn was untouched in battle with orcs in every encounter, from Moria through the final battle outside the gates of Mordor. I don’t think you give him enough credit.
In a quick round it appears Boromir and Aragorn slew 10 large orcs. Gandalf saw Uruks of Mordor, not the more typical Goblins of the Misty Mountains.
In the next round a huge orc chieftain drove through and got Frodo with his spear and “Andúril came down upon his helm. There was a flash
like flame and the helm burst asunder.”
Can you doubt what he did at Helm’s Deep and on the Fields of Pelennor?
As to Gimli, I think he grew in experience and mind swiftly in the War of the Rings. Maybe I should downgrade Legolas a bit so Gimli can have caught up to him, but Gimli semi-retired to run a kingdom and Legolas was much more active in the years following the war. Founding a kingdom of Ithilien was more dangerous than founding one in Aglarond.
Glorfindel showed no signs of clerical powers, he had some healing ability and a great ability to turn undead. The Witch King appeared to be fearful of him as Glorfindel led those that finally drove him out of Angmar. I think he fits the 1st Ed Paladin fairly well as they turn as a cleric -2 levels. He seemed to have some small magics available, including telling the future.
Elrohir and Elladan spent over a thousand years hunting and killing orcs in revenge for the harm done to their Mom. That was my main justification for their levels. It is that whole thousand plus years that gets me to rate them very high.