With all that, we're supposed to believe his constant/soul mate was Shannon and not Nadia?
again, i think it was an island thing. ...
Then we're back to the Desmond/Penny situation. Shouldn't Des's constant been, I dunno, Clancy Brown if that's the case?
maybe? i guess i'm not sure what you're looking for here. is your question "why did X wake Y up" or more "why was X with Y in the church"? the individuals waking one another up were not necessarily their true loves or anything like that. sometimes love woke the characters up, sometimes it was a near death experience, sometimes it was vaginas. seeing shannon woke sayid up, presumably, because he was in love with her on the island. seeing her was an "oh fuck, i lived on an island" type experience. same with kate delivering claire's baby, hurley kissing libby, or just libby being crazy. being a soul mate or a one true love does not seem to be an exclusive requirement for the wake up scene or the church.
if you're asking how come sayid didn't get to sit next to nadia in the church when desmond got to sit with penny, i guess there could be a number of reasons. i'm sticking with that group being an island connection. nadia had nothing to do with the island, where as penny hunted for it, communicated with it, and eventually set out to sail for it.
To the extent that a story builds up a mythology/scenario/rules and then doesn't consistently follow them (in various incidences), yes. this is one example but not the only example of the writers seemingly not keeping track of their own "rules." And if you're going to set up the final season that the entire series is not about sci-fi but about a series of magical rules you ought to at least try to follow them.
personally, i think the show stayed pretty true to itself, through out. it's a weirdo adventure with maybe-gods, maybe-magic, and maybe-dinosaurs. but the focus is always on kate getting kidnapped, hurley saying something silly, and jack crying. even through threat of time travel and smoke monsters, it's always been a series about love stories and splitting fractions.
they time travel to 1970, but the kicker is how juliet will react when sawyer talks to kate again. they're stranded on a deserted island with a plane that breaks into pieces, but there's an ongoing plot about how an old woman from the front section can reconnect to an old man from the back. there's mystical beings attempting to carve plots over centuries, but we're focused on jack getting mad at daddy. the human and character aspect
is the show. the thrill ride neato sci-fi and mystical parts are just the setting.