What year is your MPV?
What size engine?
You can have a simple combustion gas test done on the coolant to find out if there is exhaust entering the coolant. Any competent mechanic should have the tool to do this. It involves a cylinder full of dyed liquid that is placed over the radiator cap. Then vapors from the radiator are sucked through the dye and if it changes colors there is exhaust gas entering the coolant. This could explain the multiple coolant leaks and puffing radiator cap.
Okay, he is talking about replacing the head gaskets which is a long involved process. You may be better off installing a used engine at this point. Especially if the heads are sent out and one or both are determined to be warped too much or cracked. Before anything is dismantled I would have the coolant checked for combustion gases... If this test comes back negative then there is no need to remove the heads or replace head gaskets.
Do a google search for LIS-75500 to see a picture of the tester and how cheap it is!
To see an example (not your engine) of the head gasket and other components see below:
Correct, you would use the tester in the reserve tank.
I can source you one from Canada starting around $350.00 + freight or one from the US for about $600.00 + freight. So it sure could turn out to be less... but again before we go this route get the coolant tested for combustion gases.
To replace the switch by the book is a 1.1 hour job and the switch costs about 71.00