I'm rather dismayed that I may have played a part in propagating the false notion that it was the original intention.
Probably one of those times when we need to remember the 'opinions stated here do not necessarily reflect the opinions of S&S, it's subsidiaries, or CBS and it's subsidiaries".

I plan on reading some of the early Pocketbooks novels from the 1980s and like my Bantam novel reading it will be interesting to see how those early stories stack up with what we know now. And I'll be especially interested to read some of those novels where the time-frame is a bit vague. I'll be looking for little clues to see if that gives an indication on whether it was a 5YM story, intended to be post-TMP, or some indeterminate timeframe.
Your theory though I thought was as good as any as to the intent. Without asking the authors themselves what their intent was we are just left to kind of theorizing. But unless one of the authors of those novels says "yes, the original intent was an extended 5YM" then it's just a theory.
I sometimes wondered if the thought was about the Phase II TV series that never happened. Maybe some writer or writers thought that actually happened in universe. That the ship went on another 5YM after the first but before TMP and perhaps TMP was referring to the end of that 2nd 5YM. Maybe not exactly as intended for the series (for instance, Spock is still on the ship and there is no Xon or Lt. Ilia or Decker). But maybe the basic idea of a 2nd 5 YM between the end of the TV series and TMP.