Transmission must have gas pedal released to shift into 2nd gear.

What is your question?

Vehicle will redline endlessly in first gear until you take your foot off the pedal. It then shifts into 2nd and beyond with no issue.

What makes this problem better or worse?

Taking power away from the engine in first gear allows the tranny to shift beyond 1st gear.

How long has your 1995 Mercedes-Benz C280 had this problem?

First start after vehicle sat for 3 years, I believe. Problem wasnt evident then.