Interesting stuff, Tom
I have played the Eden several times, and have a routing of it in front of me. The routing is of photographic quality and shows the following.
Currently, there are only two holes which lie along the bank of the Eden, 4 and 7 (not counting 5 and 8, whose tees lie on the bank, heading inland, between 4 and 7 greens--it is a very unusual routing!). 4 is a driveable "par" 4 of 273 and 7 is another shortish hole (313), so it is hard to see what your article meant by "merging...the 12th and 13th holes." However, my routing clearly shows an NLE hole, whose tee would have between the current 13th green and the Eden, and would play to a green on the Eden, behind the current 7th tee. This makes sense, as the current 13th is clearly a better (Colt?) hole than the Steele holes which follow it. Also, it keeps the entire course to the TOC side of the railway line.
All that being said, I've spent a few minutes trying to imagine how any of these holes, under any possible old routing (including the NLE holes at the beginning and end of Colt's course) and still can't find those old 12th and 13th holes that Morrison talks about.
Any help out there?