On Mjolnir stopping before it hit Loki. I think it was likely the hammer or even Loki, unknowingly of course. One thing I notice about the panels after Mjolnir was thrown is that we never saw Thor’s actual reaction to it since his face was hidden from view after he threw Mjolnir. That might not have been all that important and might mean nothing at all, but comics are based on visuals and facial reactions are quite important when it comes to comics.
There’s also the argument that Thor threw the hammer in anger and frustration and threw it to hit Loki. Thor also knows Mjolnir won’t do much damage to Loki and that he can survive a couple of hits with the hammer, probably more and he’s never had any issues with using the hammer against Loki before. So why stop the second it was about to hit?
My guess though is Thor knows Loki is worthy, even if he’s not going to easily admit it.