Lavos showed up in a similar portal in Magus' castle...ok, didn't really show up, but since Magus 'summoned' it, I'd assume it was a similar thing.
Anyways, I think it was Dalton. You guys should read my Fan Fic I posted. It's mostly accurate, except the end. If I were to change it, I'd have Dalton going to Porre after his 'encounter' with Melchior and doing his thing there.
As for Porre's rise (it's not spelt 'Poore', people!), I think it was Dalton or someone (hell it might've been Magus for all we know =p) 'dethroned' the Mayor, as it were, got weapons from Lucca, vastly accelerated on them with help from resources in El Nido, and kicked Guardia's ass, stealing the Masamune. I'm one to believe Dalton did something to it to make Masa and Mune fall asleep, therefore leaving only MM energy (hey...Mamon Machine and Masa Mune start with the same letters!...anyway...) and his dark will to corrupt it. Or he placed a spell on Masa and Mune and "woke up their evil parts" (a la Orcha, which would explain how Porre knew how to do this to Orcha) and...yeah.
Now my theory only works, since it includes the Porre-Elements theory, in the CC timeline. Then again, that's the only timeline we see in CC that involves the evil Masamune, so who's to say Guardia even fell in the non-CC timeline? WE'LL NEVER KNOW!
...maybe.