Hi,my name is XXXXX XXXXX I'd like to help you sort this out.The picture to the left is the inside of your transmission bellhousing,between the engine block and the transmission.
The #2 mark is the slave cylinder. If they pick this up and show you,you will see the outside of the transmission.Now if the unit is leaking externally,you will get brake fluid on the inside of the bell housing that will seep out of the housing.(Brake fluid is used as the systems hydraulic fluid.) If the slave cylinder is leaking internally, or bypassing fluid, you will not have an external leak.To answer your question,yes the cylinder could fail like that overnight. As far as vandalism,the slave cylinder would be almost impossible to get to without removing the transmission but the lines running to it could be cut.If the slave cylinder was only seeping before it failed you might not have noticed any odd behavior.
As for the clutch,you want it to be inspected while the transmission is out. Usually a slave will last for the life of the clutch,so it maybe due for service.The slight shuddering on deceleration may be the indicator that it was coming due.Another part to have inspected is the clutch master cylinder.This unit is connected to the pedal via a plunger that has a piston /cylinder that forces fluid down to the slave through a tube. You need to check it for fluid seeping past the piston and back into the cab.
If you are available for the shop to call,they can show you the slave cylinder and the leak after they pull the transmission before they clean it up.The problem is most shops work on a flat rate system,so you can't keep them waiting.I think I answered all your questions, if you think of anything else,don't hesitate to ask.Good Luck,Chuck
PS: As far as price,I'd have to ask tomorrow at work and let you know.If you want the information,send me back a note and I'll check it in the morning.Thanks again