So how practical would it be to modify our new jet scrolls to instead of pushing air but tuned to push metal or admintine down the tube effectively turning it into a magic railgun of sorts?
Uh... magic railgun is maybe pushing it a bit. Magic grenade launcher, sure. But these things are not going to push projectiles with the force of a railgun. They might help to lob grenades out to medium range. But I doubt they'd go any further than that.
You know what would make a better magic railgun?
A magic railgun.2 Check Crewmen Minds: Madman, SMMI
1 Leave Them Alone: NUKE9.13
Because that worked out so well the last time. I'm sure that they won't be on guard against this sort of thing now, and would be so offended if we did it that they rescind their offer to help us. No, there's no way that could happen. Certainly not without rolling the dice first, on account of how WE ALREADY TRIED THIS.
Which is to say, even if they are lying, what of it? What do we stand to lose? I mean, if they don't want to spy for us, why on earth would they want to spy for Arstotzka? Worst case scenario, we let them go and they immediately leave the island, never to return. In which case, the same happens to Arstotzka, and no one is better off.
And if we scan their minds and detect this intention, what are we going to do? How can we force them to be loyal? We don't have mind-washing spells.
Doing this is all risk, no reward.