Council of Elrond

Well, so imagine i kill Smaug at Home in The Lonely Mountain with a company composed by Beorn, Thrain II, Glóin and Radagast (wich uses a Sacrifice of Form to slay the worm).

Well, Thrain is crowned King under the mountain and Gloin Plays the Arkenstone. Quite thematic!!

The thing comes later, when Tráin II gets corrupted and discarded, and Gloin also corrupted and eliminated. Radagast is "not revealed" (due to Sacrifice of Form), so Beorn goes alone to other Dragon Lair to seek for vengeance. Radagast comes the next turn, and rejoins with Beorn in the Caves of Ulund, they enter the place and then A Chance Meeting, Dain II comes under Radagast, plays the Arkenstone (conveniently fetched by some Smoke Rings, of course) and then comes the question... Can he be proclaimed King Under the Mountain? Reading the card's text, I see it legal, as the company is formed by the original people who killed the dragon (Beorn and Radagast)... Thoughts?

He can be proclaimed King Under the Mountain. Due to loophole in law, but still.
Beorn is the character whose presence asserts that his company is the company that has defeated an at home Dragon manifestation attack other than Eärcaraxë at Home.
For the same reason if Thráin II would return to play but would not join Beorn's company (or its continuation) he could not become King Under the Mountain. His current company would be other company than the company that has defeated an at home Dragon manifestation attack other than Eärcaraxë at Home.

This "loophole" opens the door to slaying a dragon at Home and Chance Meetin' a King Under the Mountain later in the game. There is no need for him to be present at the moment of the dragon slaying... which goes along with the book, as first Thorin and Dain after him were proclaimed King Under the Mountain, and neither of them were present at the moment Smaug as killed...

Yes.
A witness of combat with Dragon at Home, or someone who knew the witness, is necessary (and some noble Dwarf in company with any of them, but the Dwarf may join the company later).
If Beorn would leave active play, but his last company would survive, there would still be the company that has defeated an at home Dragon manifestation attack other than Eärcaraxë at Home.

That is something to think about when deckbuilding a KutM deck...

What about a returned wizard after sacrificing? Would be him able to make KutM a dwarf? Or as soon as he leaves active play looses that "witness status"?

The same what about returned Thráin II. If he will not join Beorn's company, or continuation of the company, he will not be in the company that has defeated an at home Dragon manifestation attack other than Eärcaraxë at Home.
King under the Mountain does not check whether a character was in company that has defeated Dragon at Home (at the moment of defeating, or later, or before). It does check, whether the character is (currently) in such company.

Hmmm... let's switch Beorn for Strider as the only character in the company that defeats Smaug at Home. Later on the game I switch Strider for Aragorn II. Is there any remain of that company that defeated Smaug?

Yes. After all, that is the same company, even if its composition changed in 100%.
Similarly: company with Eomer (only) faces Drake attack, then Nazgul attack. Helm of Her Secrecy is played and Eowyn joins company. Eomer is eliminated, the company does not contain any characters which was present when it faced Drake attack, but it is considered faced Drake attack this turn and now Worm's Stench may be played on the company.

Understood, and agree.