While I'm not a big fan of Teela being a clone, I do love the clone story as its own separate thing. Like Eamon, I see it as being with the blond Teela, rather than the red head, and as I have both in my canon, I'm all for it.
On the choice of why she is a clone, I agree with Scott's reasoning. I couldn't imagine her in 200X being gone from the castle long enough to fall in love and marry the man. I always took it to be a few weeks she was gone, so I never thought of there being a wedding in Panda Town. Filmation (which is obviously off limits) made it seem like she was a young girl, whose husband was off in battle with the Horde. It made sense that she was (not aware she was) pregnant, husband died, and voila, she had to give up the baby. Like Val said, if ever the birth father was revealed, it would have to be someone very huge in the canon.
Re: Marlena's bio-I'm thinking the connection is that humans will voyage out into space and colonize Eternia... so is that what happens? Marlena visits Earth and sees the start of the trek out into space? And why does she return to Earth in the first place? I always envisioned she would return home to visit from time to time, but really? Really? The implications of returning to Earth are like an episode of Sliders. The Horde coming here would be insane, in the epic way.