yeah the mechanic is right, cars no good, I'll give a a buck fifty!

Call up your local bbb or someone in DA's office and explain to them that this guy is ripping you off (veriy first ofcourse). Get away from that mechanic ASAP. Get a second opinion. Go to a volvo dealer, even they have some decency on ripping off people.

He may be somewhat right about the motor mounts failing and causing the short.

The cable (2- and 1+ wires) runs from the battery under the cross member to the drivers side. One short (- wire) runs to the body frame, and the other (- wire) to the engine underneath. The positive wire, also runs with the - wire to the engine, goes under the cross member. But (+) this goes over the cross member on the drivers side. This wire (+), runs next to motor mount and ends at the starter solenoid. It is held in a bracket, and that bracket is attacted one of the drivers side motor mount bolts. Worst case, I think in your car, the engine is hanging and may have crushed the + wire and caused a short. Otherwise there may have been a short at the sway bay, with failed mounts. The engine touches the sway bar. Both cases are very very rare.

As far a damage, all circuits are designed with fuses. I don't see any damage to the any sensors or ecu.

What heck is he replacing for $4700? Every sensor and wire in the car? No way.

Do tell how it turns out.