Personally, I DON'T accept CMI and EMI (MI3 and MI4) as canon, not just because of a different creator, but because they also seemed completely different and a PLANNED story-line was halted and something else went on.
That said, I personally believe it was all within Guybrush's head. I actually like that more, because it would not only make sense that everything in the world contained both historic as well as contemporary elements, but I love the idea of a kid's imagination coming so much to life that it's viewed in a realistic and interactive manner.
Perhaps we would have discovered even more if the third installment was created by Ron Gilbert, but now that's not an option I suppose.
And finally, I believe the Secret of Monkey Island... the secret itself, may literally be some cerebral portal to the world of Monkey Island, and perhaps Guybrush and LeChuck had discovered this portal. Maybe this is a little stretch, but it works for me.
So in that, and in all, my vote goes with the father... it was NOT a curse LeChuck put on Guybrush. It was either all an imagined adventure, or some adventure Guybrush actually went on through some portal or gateway to another existence. Dunno.
Hopefully Ron will tell us some day!