OK …

Bilbo
Frodo
Lobelia

Owners/dwellers of Bag-End?

Close, but not correct.

Bungo, Otho, Sam and Frodo Gardner would also belong to the list of owners.

And dwellers would include all the above, plus Belladonna, Rose, Lotho and all Sam's children.

I am going to change this slightly, back to the version I originally made:

Bilbo
Frodo
Sam
Lobelia

Ordinary folks who were nonetheless able to stand up against evil?

(I will always remember how Lobelia beat up the ruffians with an umbrella)

I like your reasoning, but so was Tom Cotton, to name but one. And were those 4 hobbits really ordinary?

....

G55 was closer.

EDIT: I agree with you about Lobelia. It's nice that she was redeemed and kind of liked in the end ... despite what happened to her son. :(

Owners of Bag-End who gave it away while alive (i.e. rather than dying and it being inherited):

Bilbo -> Frodo (when he left after The Party)
Lobelia -> Frodo (after she was freed from Lockholes and returned to her own people)
Frodo -> Sam (when he left for the Havens)
Sam -> Frodo Gardner (when he left for the Havens)

Formendacil is right.

This was my original idea, which I changed to people who gave Bag End to someone who was not a direct descendant (and left it whilst still alive), but I decided that was too convaluted, and changed it back.

Over to Formendacil!

Hmmmmmm...

Eldarion, Elboron, Eldacar, Galador

So. Two of these are children of an elf and a mortal (Eldarion, Galador). Two of them are the last named members of their lines (Eldarion, Elboron). All four of them are princes, but that doesn't seem specific enough.

Two of them are the first heir to a line - Eldarion to the house of Telcontar, Elboron to the Princeship of Ithilien. Galador is the founder of Dol Amroth, but is also the second known Prince of Belfalas (after his father). So are they all the second known to hold their title in a specific line?

Let's see what we can do with Eldacar. Hmm... since Isildur claimed the title of High King, Eldacar of Arnor (despite being much less exciting than his later southern cousin) was technically the second King of Arnor, after Valandil. So one possible answer is that they're the second known holder of/first known heir to their various titles.

Another answer, this time looking at Eldacar of Gondor, is that they're all children of two races. Eldarion and Galador are Elf-Man crosses; Elboron is Gondor/Rohan; and Eldacar of Gondor is son of a Gondorian father and a Northman mother (actually the same as Elboron, give or take a major migration). So that could also be what you're thinking of.

hS

Your last formulation is closest, though I have a more specific fact in mind--i.e. Earendil would NOT share it.

They're all boys whose mother was of a different race than that they are held as belonging to. All four were very much their fathers' sons.

I can't think of anyone else who meets that precise description - all the candidates fail one way or another (assuming Dior is taken to be Elvish).

hS

You're on the cusp of it. Specific to their shared mixed race situations, what do they have in common?

Their fathers were all descendants of Numenoreans and their mothers were not?

We have a winner!

(Specifically, I had it phrased in my mind as "children of Dúnedain fathers and non-Dúnedain mothers--but that's just the rose by another name).

On to Urwen.

Turin, Eowyn, Sam

Hmm. Turin and Sam both stabbed monsters by getting underneath them, but I think Eowyn chopped the Witch-King's steed's head off, so probably not that.

hS

You're close.

Both Sam and Eowyn were protecting someone when they went all monster-slayer, but... I feel like Glaurung just happened to be wandering past Brethil when Turin killed him? So not that either.

Was Eowyn using her own sword when she fought the Witch-King? The others weren't.

hS

Well, for all intents and purposes, Gurthang was Turin's sword, for it was remade for him. And he was protecting his wife/sister along with entirety of Brethil inhabitants.

But no, what I had in mind is more on the general side of things.

But not, I assume, as general as 'they're all monster-slayers'?

hS

Eh.....:o

In my defense, I just woke up.

I mean... they all killed non-humanoid monsters with swords. That's actually pretty specific, when you get down to it. Balrogs and trolls are humanoid, Smaug died to an arrow, Ungoliant isn't ever said to have gotten stabbed...

hS

You're up.

Um, okay, let's try this:

Feanor
Eol
Tar-Aldarion
Tarannon

Probably pretty easy.

hS

Their wives were estranged from them?

Yep, simple as that. Back to you.

hS

This one is pretty easy too. :D

Eldacar, Zimraphel, Helm Hammerhand

Oh, they all had their thrones nicked, didn't they

hS

Yea.

Okay, so let's try something tricksy:

Fastred of Greenholm
Lily Brown
Bell Goodchild

... and no other named characters.

hS

They are related to Sam, either via blood or marriage.

They are indeed. So are a lot of other people. :)

hS

There is very little known about them, and I can't think of anything else.

I'm afraid that's not the answer either. :) But between the two you're kind of edging closer.

hS

They had famous spouses?

Not really, but again, you're sort of edging closer.

hS

I am befuddled..... *shrug emoji*

Okay, so: Urwen, your three answers together almost (but not quite) hit the mark.

-They're all Sam's relatives (how do we know that?).
-We know little about them (how little?).
-Their spouses are better known than them (why?).

There is a specific document which is key to this one.

hS

They were all mentioned in Red Book of Westmarch?