I thought Dark link had something to do with the interlopers as well (at least that was seem in Twilight Princess.)
But Link is suppose to be pure of heart right? So if Demise helped create dark link that might also imply he, meaning link, held such a similar darkness inside of himself in a way. Really that is the whole reason dark link may exist (as a way to fight ones self, metaphorically and all that jazz), but if the hero did posses such evil inside himself wouldn't that mean he couldn't seal away Demise in the first place?. He couldn't have properly made such a wish to the Trifroce (usually most "evil" people ARE corrupted (such as by greed) so Link would have ended up (probably unintentional or subconsciously) wishing for something other then the end of Demise (such as perhaps getting some action from Zelda finally? or Ultimate Power for himself - much as how Ganon would have?).
Nice critical thinking exercise and an interesting concept but it seems to snuff itself in the end.