Excellent work!
My car the 1989 300e, also had a similar problem,
In the mornings, when the engine was off for 8 hrs plus, I would try to start it... if it was feeling good, it ran at 400 to 500 rpm cold... usually would stall out... I would have to sit for 15 minutes while my engine warmed to 70C+ just to keep it alive... in park... putting it in gear was a different story..
I looked at my ICV first and tried cleaning it with carb cleaner, lots and lots of brown juice came out, really sick...
When is was clean, I suspected the OVP just like you, at $50+ it was pricey... but got the new one installed...
Eureka!
I got that feeling, I fixed it! I fixed it!!!
NICE WORK!