How is Thanos able to hold almost all of the Infinity Stones barehanded?
No answer is given in movie itself but this is what The Collector said about stones :
"These stones, it seems, can only be brandished by beings of extraordinary strength. These carriers can use the stone to mow down entire civilizations like wheat in a field. Once, for a moment, a group was able to share the energy amongst themselves, but even they were quickly destroyed by it." ― Collector src
It was never said that nobody can touch Infinity Stones barehanded. And we have not seen many powerful people even trying. Ronan touched power stone briefly before placing it on his hammer too.
Is it because of the fact that he is an Eternal with the Deviant Syndrome? Does he have the same origin as Comics?
We don't know, we can make wild guess for now, we do know that his father's name is the same:
"Welcome Thanos, son of Alars." ―The Stonekeeper
And then this is what director's said:
We were considering showing [Thanos'] backstory, but ultimately felt it wasn't necessary for the movie. - cinemablend
And we almost got a tie-in novel called Thanos: Titan Consumed but it became non-canon now:
Hey all - thanks for all the shares on this. Unfortunately, despite being told the novel is canon, the information given to me was INCORRECT. I'm really sorry about this. Article being fixed asap. - twitter