I think I had a walkthrough at some point, but I can't find it now, and I don't remember the solution. I wrote it 10 years ago! I seem to recall that all the endings are possible from the moment you enter the palace at the end. You basically need to find alternatives to killing the king, and if you search the palace enough, you should find solutions.
One I half remember is as follows:
SPOILER
On the ground floor there is an amethyst eagle amulet that will grant you the ability to read any language. Somewhere upstairs there is a sorcerer's study in which you can find a spell, which you can read if you have the amulet. Then you need a unicorn's horn. Then you put it all together at the end.
So there is an article in Ryan's Gig Guide about the numerous times Diamond Head have been invited on stage with Metallica. There was a comment about how if there wasn't a Diamond Head there wouldn't be a Metallica. And how if there wasn't a Metallica there wouldn't be a Diamond Head second career phase.
I've encountered what I assume is a programming bug.
Halfway through the adventure, the "USE" button disappears from below my inventory. This makes doing things like
SPOILER
finding the hidden locker
END SPOILER
impossible!
I have continued into the Golden Crate from the previous installments, if that makes any difference.
I've fixed it now. The USE button is meant to disappear at that point (to stop you switching weapons) but it now returns immediately after the DOG-BOT fight. Thanks for pointing this out.