My first thought would be if there were to be a problem on the alternator ignition side (purple wire), you should in theory see your volt gauge up over 14.5 volts. I know you said you disconnected the oil PSI switch but for how long? The engine would run for a bit at idle even with no fuel pump since the remaining fuel needs to be emptied inside the carb. As one retest, you may as well pull the oil PSI switch connector while watching the power at the fuel pump to see if it goes off with engine running of course.
As you will see in the wiring diagram linked below, the purple wires first point of contact from the alternator is the oil PSI switch then it turns into the purple/yellow wire then onto all the troubled components of the white wire. But it cold also be #14 (R starter solenoid).
The TKS service manual is small enough to read most of it electrical wise and just helpful enough to help guide you through this as long as you have a clear opened mind of how the system works of what to check. I think you do.
http://boatinfo.no/lib/mercruiser/manuals/mercruiser41.html#/28
If the above is not the case, then as a starting point, the amplified voltage may be coming/starting from the alternator #22/23 going into the 20 amp fuse and working it's way to the diode.
Since you can pull the diode, TKS module and temp switch and turn the engine off, try pulling (#20) 20 amp fuse to see if engine and/or fuel pump turns off, then disconnect the red/purple wire connector on the alternator. Also check voltage everywhere you can/disconnected.
The orange wire off the back of the alternator goes down to the starter. Is that where you read the alternator 14.5v?
Be 100% sure all wires are clean and tight starting at the battery heading to the starter and up to the engine. Also check all wires under the dash and ignition key. You also need to know what voltage you have at the back of the ignition key (all three or four wires?).
Don't use wing nuts for the battery cables either.
