The version I had had the standard card with directions for installation that came with every game I've ever purchased that came in a box. Just about every RPG came with a little manual, allegedly written by some old, wise person, that told how to start a game, create characters (for games with variable characters--some were entirely pre-cast,) and what keyboard/mouse commands did what, plus a few charts, possibly with a list of spells, another list of monsters, and ones for armor and weapons. I don't recall directions for potion-making in MM6, except possibly for the basic 3. I know the only map included was the one for New Sorpigal, which was quite complete.
I believe the truncated directions were possibly a sweetheart deal with Prima--a sort of You scratch my back, I scratch yours. The Prima manuals contain ads for other games in whatever that developer's library. I can imagine the developer then refraining from giving away too many free hints, so that players would then buy the Prima book (and see ads for other 3DO games, in the case of MM6. There was at least one postcard-size ad for a 3DO wargame--Plastic Soldiers, perhaps?)
Your description of the Strategy Guide sounds intriguing.
Incidentally, the only discrepancy I found in the MM6 Prima Guide was the inn in Mire of the Damned mislabeled as The Haunt, instead of The Rusted Shield Inn