#1 - The pushbutton controller in that year was just a switch board. The electronic were in two separate controllers behind the glove box. They are somewhere in size between a paper back novel and a pack of cigarettes. The Temp Controller is in a silver metal can and the Blower controller is a black plastic box. The circuit to the mono valve passes through the switches of the pushbutton controller into the Temp controller except in Min and Max positions which are delt with in the PBC.
#2 - Your impression of pressures are wrong. Proper pressures on the "Cold" side (low pressure) should drop to around the high 20's psi. The high side will be above 200psi and be dependent on many things including the ambient air temp.
The oil in the compressor/system is best left to professionals. It really shouldn't be delt with except with major repairs. You can not add oil to a pressurized system without special tools and training.
#3 - We usually get two hours to replace both lines (the only way we do it). You have eyes bigger than your fingers if you think its a half hour job. I doubt you will like the job.
#4 - We use 20w50 oil all year round here in Florida.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician