Sounds like a few different and unrelated problems. I would try to tackle one at a time.
You've already got some good advice on the refill problem. The probe is likely too low, which is causing the low fill level. This probe is easily adjusted. It is on top of the gauge. As others have said, taking apart and cleaning the brass distribution block may fix the issue with the over pressure valve. This is the big brass thing in the front left bottom of the machine. All of the replacement o-rings and seals from that part can be found at Espresso Parts.
Second, either your pressurestat is set way too high or your emergency shut off is defective. You can buy a cheap gauge somewhere to check the boiler pressure or just buy a new emergency shut off thermostat.
In cases like this, it is usually better to take the entire machine apart and do a full rebuild.
Here is a link to manuals:
http://iacoma.cs.uiuc.edu/~paulsack/faema_manuals/