Guardians are already in the game.
I think Dartz is the only one significant enough to maybe become playable. (Knowing that Rafael defeated freaking Yugi/Atem!) But tbh the W/L rate is something I've looked down on most protags for because an invincible protag is boring, and not every duel should require the ultimate stakes.
They are canon. The anime and manga have SEPARATE canons. There are events that happened in the manga and not the anime. (Death-T and Pegasus dying being the biggest examples.) And Duel Links utilizes both.
The ''Waking the Dragons'' arc is the 4th official season of Duel Monsters series and all the events and characters are 100% canon and part of the franchise.
Pegasus died when Yami Bakura killed him, but that's all we know. We don't know how did he killed Pegasus, or whether the Eye was taken before Pegasus is dead or after.
Death-T is basically a series of lethal/deadly games like shooting game or rhythm game, meant to humiliate and break Yugi and his friends, with the event culminating in a card game of Yugi vs Kaiba.
And when I said "lethal/deadly games", I meant those games are normally safe, but is rigged by Kaiba to be dangerous for Yugi and his friends, or at least seemingly dangerous.
Kaiba: As president of Kaiba Corportation, I declare your move, INVALID.
Dartz: Grr!! B-But KAIBA!... Y-you... can't !!!
Kaiba: As a matter of fact, Dartz, I do this every day of my life.
Dartz: Go ahead and summon whatever you like
Kaiba: Fine, I summon Obelisk the Tormentor with no tributes at all and his attack power rises to... 243921749021823 ∞ ∞ ∞ !!!