When did Barnabas become so attached to Maggie as Maggie
That's an excellent question - especially considering that Barnabas was totally willing[spoiler]to let Maggie die when he decided she had to be used as the lifeforce for Eve.
[/spoiler]But then, Leviathans isn't exactly a storyline that's concerned with little things like storyline continuity, logic or fact. And sadly with respect to that last one, the next ep features one of if not the most ridiculous plot point ever within what has to be one of the most if not the most ridiculous plot turn ever.
But more on that in the discussion for Ep #952...
If Dark Shadows hadn't been canceled, and the "No more vampire" clause in Frid's contract (or whatever it was) remained in force, would the fact that the Barnabas Collins character had once been a vampire be an arcane bit of Dark Shadows lore rather than the part of Dark Shadows most likely to be remembered by non-fans?
There wasn't actually a clause in Frid's contract that he wouldn't play Barnabas as a vampire - he simply said that he didn't want to play Barnabas as a vampire anymore. What was in his contract, though, was that Frid had to be given a character other than Barnabas to play. And frankly that was only fair because every other major actor on the show had already played multiple characters by the time he'd insisted on that.
However, you bring up a very interesting question - and I'll take it even one step further: if DS hadn't been canceled and was still on the air today, would Barnabas even be the character that's most associated with the show? Every long running soap has had characters who at the time they were on the show seemed so identified with the show that it would be impossible to ever think of the show without thinking of them. But funny thing, when they left those shows, yes, there may have been gaps in fans' hearts, but the shows soon introduced other characters who eventually filled those gaps. And while the departed characters have most probably remained in longtime fans' hearts, they're no longer seen as irreplaceable or as the character most identified with the shows. And when it comes to newer fans, many if not most don't even know who some of the classic characters on their favorite soaps were or even care about them because they're so far removed from what happens on the soaps nowadays.