Yes, he is the guy you give your sibling to in the deep roads to become a warden.
To the topic itself, the idea is very silly and far fetched so people are going to poke fun. It's like saying the blonde elf woman in the concept art is origin city elf's mother Adaia(sp?) because omg they are both rogues, both elves, both female, and we never saw what Adaia looked like! The reasons people immediately dismiss the notion that the new companion is "Grigor or Greagor" are:
-He was one of the Ferelden wardens and every single one of them died except Alistair and the PC who would have died too if Flemeth and Morrigan didn't need their newly tainted selves to make a baby.
-He wasn't even a character and was only mentioned to give life to Alistair's story as was the elf warden that kept to himself. BW only resurrects characters they feel add to their story, they would have no use for resurrecting a one line blurb who wasnt even in the game when they could make a new character.
-Alistair described him as being quiet, calm, reserved. In the art of the group at the tavern he is obviously laughing loudly and strongly.
-Alistair described him as having the biggest bushy beard you had ever seen, the warden companion has a normal beard no crazier than Duncan's
The whole thing is just silly.
Um the City elf's mother is confirmed dead and before you say this guy is no he isn't he's just likely to be.
When we meet Riordan in Origins it is a surprise to our character that he didn't die at Ostagar. So already we have a warden who didn't die at Ostagar and there was an explanation for that so another one not dying is perfectly plausible.
I would also add that the Wardens do send people to other countries sometimes like in Awakening for example. He could well have been sent somewhere else for some reason.
Him not being a character is irrelevant and as we discussed they don't need to resurrect him as he may not be dead.
He could easily be quite, calm, reserved ect when sober and different when he is drinking everyone under the table (I know I am) and the beard thing could be an exaggeration or maybe he trimmed it.
This idea is very very unlikely but it's not silly, your reasoning on the other hand is so shh