Intriguing. I sent it to my mate (he initiated this whole business) and it worked for him, then I tried it once myself and it worked again, but now it's no longer working again. I've closed down clickteam and tried again in case I get one shot. I've reverted to exactly the way it was when it worked, but the tankette doesn't exist, nor does it exist when I close and reopen that exact file and try to run it again.
My version is a copy/paste of yours, and the turret rotation is still crazy. This isn't a big deal because the invisible mouse object solution is admittedly less elegant, but it works.
Always: [TURRET] Set diurection to ATan2(YMouse - Y( "Turret" ), X( "Turret" ) - XMouse ) + 180