Well I'm from a different universe, but from what I understand, Mjolnir isn't "heavy". Mjolnir is a fixed quantum point in the universe, with some level of sentience. Only those who are worthy can move Mjolnir. (Well, gravity can too, but the gravitational effects act as though it's no heavier than if it was made of steel. This is why it falls through the air, but doesn't just go through the floor.) So the question isn't whether it would be heavy in Tony's hands, but whether Mjolnir will acknowledge Thor's indirect movement of the hammer through Tony. Most likely not, because Mjolnir is incredibly stubborn.

I'm agreeing with Barbara on this one. It all comes down to whether or not Mjolnir can read Thor's intentions, and if it'll choose to actually go through with those intentions. But hey, if Thor can't throw Mjolnir he can always just throw Tony instead.

We were just wondering, if Tony can hold Mjolnir but Thor holds Tony and then Tony tries to hold Mjolnir would it work or would they both just collapse cause Tony doesn't get to wield Mewmew? But that answer was very insightful, Barbara, thank you.