The miss after a few minutes idle time sure sounds like a valve seal, but your oil consumption says that this is not a huge issue. Do the valve seals and check the guides when you get a chance.

If it's not flooded when it won't start, the next thing to check is if it's not getting fuel. The best way to check would be with a fuel pressure gauge hooked up to see where the pressure is while cranking but not starting.

An easy way to get an indication would be to spray something flammable (carb cleaner, WD-40, or similar) into the air intake while trying to start. Look for better, worse, or no change. If this improves the condition, or even starts and runs for a few seconds, it indicates a lack of fuel in warm starting conditions. I'm not sure of the specifications for residual fuel pressure (pressure that the fuel system is supposed to maintain after shut-down), but it sounds as if the engine won't start until pressure builds up in the fuel lines.

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1989 300 SEL that mostly works, but needs TLC