Ah, the sweet taste of success, if only I had looked at the problem from another perspective sooner.
The issue with the Yashinon-DX lens was with the rear element moving too far back into the focal reducer when focusing to infinity, making contact with the reducers glass elements themselves. I couldn’t move the rear element of the lens forward and in my frustration didn’t look at the focal reducer properly.
To the eagle eyed out there you will have spotted the solution already – simply move the reducing element back within the adapter!
This is done by screwing the focal reducers glass element within the adapter either clockwise or counterclockwise depending on where you need it to go. It’s a little bit stiff but moves smoothly, and will definitely stay in place once in position. Mind blown!
The end result now is a Yashinon-DX 50mm f/1.4 lens that can focus all the way to infinity whilst being used with a focal reducer to gain a little more light and picture from my crop sensor. The one thing to be careful of is this will not work on a traditional DSLR as the element for the focal reducer now sits quite far within the camera body, no problem with a mirrorless camera but a caveat nonetheless.
The solution couldn’t have been simpler, if only I inspected the issue correctly the first time around!