strip 68 or so) planned to address the difference between Elan's and Nale's upbringing, and that would have been rather difficult without Tarquin coming into play at least for a while. What the contemporary audience expected when the prophecy about happy ending was made doesn't matter at all.
What do you mean by "Xykonishly evil", exactly? Is it "completely devoid of any even remotely positive morale traits"? Or is it "Evil boss of a group of antagonists"? Or "beyond redemption"? Or "one who strives for the ultimate power at all costs"? Or something else? Sometimes it is hard to understand Buffyishly phrased statement.Tarquin is Xykonishly evil.
Imagine this (still quite plausible) scenario:So there is no way Tarquin (and Nale) can possibly contribute to a happy ending for Elan.
The Order, the Guild and Team Evil battle at the Girard's Gate and it ends up being destroyed. Tarquin survives. Tarquin learns that Nale was deceiving him about Xykon and ritual, so he refuses to pursue the Gates and stays in the desert, ruling his Empire. The Order and the Team Evil start another race to the fifth Gate, where the Order finally destroy Xykon once and for all and end the Snarl menace to the world.
At this point, as you suggested, it will be okay for Elan to have his happy ending - he'll settle down with Haley, raise a few kids and let "the story of the Order battling Xykon over the Gates" leave them for good. He'll think it's a sad thing about his father and brother, but Therkla's death was a sad thing too and life goes on. Do you think it may well end up this way?
Because I think that would be a display of shallow morality on Elan's part.
He is not going to leave the suffering continue. He will fight for the freedom of EoB citizens immediately after the fight for the very existence of the World is over. And he already has a plan, and he seems to be really keen and happy about it which, considering his Good (most Good mortal character of the story) nature, implies non-violent, or at least non-sadistic handling of his father (Elan did state it was only "sort of" a defeat).
Now, Elan has grown quite a bit over the course of the story. He is still goofy, but not moldy-carrot-box-dumb-goofy anymore. If it turns out that his Super Top Secret Plan is utterly out of reality it will be underwhelming, to say the least. This will be his moment of triumph - his plan must be at least plausible (whether it will actually work the way it was intended to is an entirely different matter).
That is why I believe it is possible for Tarquin to be redeemed, which puts him in a different light than Xykon, or Belkar.