Thanks for clarification!
Regarding explorer boats -- is their inability to merge documented anywhere where I can see in-game? I'm just asking to know where to look regarding other units that might be unable to merge.
Also, one more question -- I'm in Ancient Era now, and, considering my post above, I was indeed able to merge three 1.5x obsidian macemen into a 2x obsidian maceman. However, I wasn't able to do the same to three 1.5x Horseman units. Why?
Update: I was able to combine them, when I got the three of them fully healed
I believe all units must have all of their movement points to merge. By the way, three 1.5x units make a 2.25x unit, not 2x... they increase by 50% strength each merge. So, for example, the atlatist has 4 strength; merge 3 of those, and the resulting unit has 6 strength; merge 3 of those, and the resulting unit has 9 strength (1.5x 6 strength, 2.25x 4 strength).