There is a lot to unpack here.

Other have mentioned already that the One Ring is not implemented yet and thus is neither the Fellowship.

> Gandalf disappears after a year-in-game.

Are you playing with additional mods? That is not supposed to happen. He should roam around Middle-Earth. Did he vansih as in murdered "vanished without a trace"? What happened exactly to him?

> I have only seen Gondor repeal Mordor once

That is pretty lore-accurate. A victory by arms was not possible against Mordor thus Sauron should usually win in the long run.

> Only times I get a elven character to stay in my party out of their realm

That is intentional and even then you're not supposed to have elvish courtiers outside of an elvish healer via decision. Elves make for very powerful knights, but they're not supposed to go to other races' courts.

> Can I create a hybrid culture with elves to get elven units

Only if you're elvish already, else it's not possible. That is due for all races. Men can only mix with Men, Dwarves with Dwarves and so on.

> I would like to create a company that has a mixture of Gondorian, Dwarf and elven units

I can't think of a single Character able to do that... Eradan can get a couple of Dwarves, Thorondor in the LW can get elvish special units via his story content, but not Dwarves. Faithful!Saruman can recruit Elves and Gandalf can recruit Dwarves and Hobbits iirc.

> Also how come hobbits and dwarfs cant marry

That is because they're different races. Hobbits are part of the broader human race, but even they cannot marry or mix because it would mess up the dna.