I think that this has been overlooked & that we could fix it with simple character expansion.

In one of the story versions BUM & BUUM were two different characters- you first encounter BUM & then you have a final fight with his boss, BUUM.

Maybe we could have SUM(Small Unfriendly Monster) & BUM (Big Unfriendly Monster)

If you look at it, that could potentially solve this problem. SUM would be the kidnapper, player would have his first fight with SUM in mansion, then he would fight with BUM in the castle and, when BUM escapes, the final battle would be at the swamp, with BUUM (BUM high on chemicals). Swamp & swamp castle are very important parts of the story since they give a clue to the main villain origin as a frog & once BUUM turns to BUF you know that everything is over.