My 1983 300SD got stuck while I was parked at the pumps filling up.

After having it towed home.. thinking it was the tumbler/lock that
was keeping it from turning it turned out to be the steering column lock
causing all the problems. At least for me.

I got to your stage and got that stearing column lock pin out of the way
and this allowed the key to turn once again, I removed the collar.
Removed all the guts related to steering column lock.

There is a nother thread on here about this process.
I rememebr the tag line being smooth as butta' - describing
how easily the ignition turns once all the stuff is removed.

Worst case scenario: An Ebay ignition housing for $40 and a
dealer ordered tumbler assembly that matches your VIN for maybe $80.

You are heading in the right direction though trying to get that key to turn just once
so you can get that collar off.
But see if wiggling, pulling, pushing the steering lock pin doesn't free that up.

__________________
80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250