I'll try hunt one down for you. I'll have to find the old source!
Lamb joined a forum to bitch about something. Who would have thought?!!?! I stand by that decision too, it was copy protection and it sucked. Sorry if I had a copy of the game growing up that had a shitty manual that was wrong, that may have tainted my view of that crazy bullshit spell system!
It is one of those grey areas, KQ3 had two forms of copy protection. The traditional disk based kind. I.E. the disk was supposed to be difficult to copy (obviously ways to get past that).
Also technically wasn't a very good 'copy-protection' since it made up 90% of the game's actual puzzles, was important to the story, and pretty much all the official hint books included copies of the spellbook. I.E. The Official Book of King's Quest by Donald Trivette, and The King's Quest Companion by Peter Spear. Basically someone could buy either of those two books and actually pirate the game! Of course in Sierra's discussion of it, some argue it was a gameplay mechanic, and not actually intended to be copy protection it was one of "Roberta's great ideas'. However, some argue she intended to actually make it copy protection. So who knows...
From memory @daventry I'm pretty sure that cutscene happens if you wait for Manannan to leave the second time. It's certainly when he leaves for a trip once.
I'd have to check the notes I put on the wiki, but I seem to recall it might by third trip. After you have used up two of the food items. But you might be right as well.
Also, another change I just remembered while typing that was we had a limit on how many times Manannan could go away before you die. This was to remove the dead-end where you can feed Manannan all your food and then not be able to progress.
Technically there is a limit in the original game. Once you run out of food you die, he will kill you upon the 5th time he asks for food (or at least the second time you refuse to give him food (this could make it six times he asks for food), assuming you haven't received the minor 'warning punishment' first earlier in the game).
At least one of the remakes does away with this, by adding a never ending source of food at the Bear's House IIRC.