I'm having a similar tension release problem on my 1916 Singer 66-1 and would love some help please, because I haven't been able to solve it except by making a longer pin. The original pin is 1" long and slips through the stud like greased lightning. (I took the whole tension assembly out and apart and cleaned and polished it.)
If I put the tension release disc on the stud the wrong way round it works (that is, the "bar" is closer to the machine, and the convex side faces towards the beehive spring) but then it is very hard to get the top tension loose enough without starting to wind the nut off the stud.
If I put the check spring regulator further into the machine enough to make the tension release work then the set screw is well proud of the machine (over a thread clear), it doesn't seat in the "flat channel" on the regulator and the spring will check down on the first stitch and then get stuck because the tension discs are free to press up against the machine head.
I made a short video to show the problem because I cannot figure out what it is.
https://www.youtube.com/watch?v=qUvIYHNixTE
Annoyingly I had exactly the same problem on a 1950s Singer 99 I spruced up for a friend - it was the only thing on the machine I couldn't solve and I had to use the same solution. So 100% of the 66s and 99s I've looked at had this problem. :-)
Thanks heaps... Megan