Why on earth is the dealer using a generic reader? If you had to pay them for their time (I'm sure you did) they should have a volvo scan tool to tell you the exact codes. The 300-301 codes don't tell you a whole lot since you're running much too rich.

Anyway here's the list of causes for 301 (from Volvo):

-Defective spark plug
-Blocked / leaking injector
-Uneven compression
-Air leakage
-Faulty fuel pressure
-Low fuel level
-Water in the spark plug wells
-Contaminated or incorrect fuel
-Repeated cold starting where the engine coolant temperature (ECT) has reached normal operating temperature between starts
-Poor fuel quality (summer fuel in winter).

I don't know how they got the fuel pressure numbers. 396 is just slightly high (line pressure should be 375-395, so 396 is within gauge error, probably). Check residual pressure, turn off car and wait 20mins if the fuel pressure is <200kPa you've got problems..

Are ALL spark plugs the same? If not, try switching ignition coils to eliminate any of them from not sparking which causes this.. If so, what is fuel trim at? If its been enriching the fuel mixture a lot then you have O2/MAF/Vacuum leak problem.

AFAIK a 'failing' fuel pump that is putting out too much pressure comes from the pump not knowing when to shut-off, meaning it keeps building up pressure (the pump can put out 450+kPa). However, it isn't as common for the 'shut-off' to fail as it is for the pumping mechanism to fail..
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If you're not driving it "like its stolen," are you really driving?