TheVaultDweller
Front line cannon fodder
I thought he would have had to return Mjolnir to its point in time, similar to the Stones. Otherwise, that Thor won't have it for his battle with Malekith and everything else that follows. Which would be a super dick thing of this Thor to do, even if it doesn't screw up the timeline in other ways.
Because even if we go with the "every change just makes a new timeline" thing, it would potentially create a timeline where Thor fails to stop Malekith, not having Mjolnir to aid him in the fight, resulting in Malekith succeeding and blacking out the entire universe in that particular timeline. So, yeah, a dick move to say the least.