do you think Dean will go easier on Jack once Cas is back?
Well, Cas’s death is the elephant in the room now and Cas coming back will obviously change many things.
I don’t really think Dean is hard on Jack solely because of Cas’ death, though. Sure, he looks at Jack through a lens of attribution of responsibility on some level for Cas’, Crowley’s and (supposedly) Mary’s deaths, and he generally is looking at the world through a lens of grief and depression that makes him believe that they can’t get a good thing, so his hopes that Jack might not turn out a huge danger are very very thin.
But I think that Dean is not just looking at Jack through the lens of Cas’ death, but also through the lens of his own past experience, especially in regards of the Mark of Cain. Season 10 was all over 13x02 in multiple ways, although there’s also hints that Dean is nowhere the same Dean he was back then (compare the Black Spur bar at the beginning of season 10 and the scene in the bar also called Black Spur now; see Dean in 10x09 making a weirdly contradictory apologia of John, now implicitly admitting that his seeking of John’s approval wasn’t really worth it).
Anyway, I don’t think that once Cas comes back Dean’s fears regarding Jack suddenly disappear, because I think it would be reductive to attribute his attitude towards Jack solely to the grief and anger due to Cas’ death. I see the last scene of 13x02 as the two of them coming to a bitter but honest understanding of sorts, because Dean sees his (fairly recent) past self in Jack, having some ancient dark power in him that he can’t really understand and that escapes his control as soon as he’s upset or scared or in pain. I think that Dean is wary of Jack because of reasons unrelated to Cas - he has an antipathy for him that precedes Cas’ death and was definitely deepened by the events of that night, yes, but now he’s gotten to know Jack better and he can see that he’s not evil “personally”, so to speak, but he’s something dangerous and potentially evil in him that could turn very, very bad, and it’s possible that Dean sees that as even more dangerous than just some devil-like dude, because Jack is innocent, and innocence plus dark ancient power is a scary combination.
(I’m on team “Dean isn’t being stupid or mindlessly stubborn”. In fact he’s allowed his perception of Jack to change already, although it’s subtle.)
Once Cas is back... it’s possible that jealousy is going to flare up. A more dramatic version of “since our child was born, my spouse only cares about the child and no longer about me”. We’ll see :D