1. Go back to original spark plugs, Bosch H9DCO I believe it is, .032" gap. The problem with Platinum plugs is that they are typically resistor-type plugs. The plug wires on a MB have the resistors built into them, so you are running double the resistance in the plug wires, which is a bad thing. This could be the root cause of your driveability concern. The 3 mile trips don't help any, and I would still think the valve stem seals may be a concern, but again at this point may only be the cold-start "false start" problem.
2. A 91 300e has very little to offer in terms of on-board diagnosis. It won't pick up on the bad Overvoltage Protection Relay. Again, there is a TSB on/around this year. The OVP should be examined to check the part number and country of manufacture. If you want I can post the "bad" info for you.
3. Be aware if you use a battery cut-off switch, you will need to recode the radio to have any tunes while driving.