Although I wrote an article for Bob Monaghan's site obout the Sekor lenses for the Mamiya TLR, I do not consider myself an expert.
The 180mm Sekor is not a true telephoto lens but resembles the old 1920s Ernostars by Bertele. But there are significant differences including: the cemented elements in the first group are reversed, the second element of the Ernostar is a cemented doublet, and the final element of the Seiko is a planar meniscus.
Bertele designed the Ernostars when working for the Ernemann Company. When the company was taken over by Zeiss Ikon, Bertele began work on an improved Ernostar design. Later still Bertele used the improved design as a basis for the famous Sonnars. So although the Seiko can trace a pedigree with the Sonnars, they have very little in common.
I have disassembles my 15cm and you are correct, it is easy to reverse the elements. From what you wrote, it seems that you have correctly diagnosed and solved the problem. Yes other elements could be reversed, but that rear element is the most probable. Your reverse-logic method, deliberately producing the error is a good play. I don't think you should have any more problems.
If I can be of further help please feel free to email. truly, dr bob.