(1) My biggest question: How can a human being be a mythological bird?
Many centuries ago, there was a huge controversy among Christians as to whether Jesus started out divine, or if he was born human and got promoted. I assume people died because of it, because that's the sort of thing that went on back then. Are you sure you want to pursue the question of Laura's exact nature?
(2) Does Laura *have* to go up in flames? Why, if she's not old and in need of regeneration to survive?
I recommend to you "The Deacon's Masterpiece", a poem by Oliver Wendell Holmes, Sr.
(3) How often does Laura have to burn? If every 100 years, then why did she burn in her apartment in Pheonix, Arizona, just weeks before this storyline?
The fire in Phoenix makes no sense to me. But maybe the Dark Shadows phoenix of 1967 acquires energy through being burnt, whenever it happens, but the fire that happens at the hundred-year mark is the jackpot fire.
(4) Does Laura actually go to another world after burning?
I believe that she believes she does. If in fact she doesn't, and if after at least two previous century-mark burnings she doesn't know that she doesn't go to another world - then - oh, dear, that's getting into deeper territory than I care to enter at the moment.
(5) What happens immediately after she burns?
This is related to the previous question. But if Laura is immediately reborn into our world, with no intermission in paradise, then what is wrong with that? Life is good even if you're not in paradise.
(6) Why the deadline?
Because that's the way it is in the Dark Shadows universe. If you don't get home by midnight, your coach turns into a pumpkin. If you don't burn up every hundred years, you lose your eternal life.
(7) What role was David supposed to play in this process?
It seemed clear to me in the fishing shack fire that David's presence was not required for Laura to do her 100-year burn properly. But the story says the phoenix builds a nest before igniting, so apparently the maternal aspect of the phoenix was important. Maybe there is a strong, superhuman connection between the phoenix and her child, so that she feels he must be with her in the fire, but at a certain point, if the child is not there, the connection is destroyed. I'm having trouble working the free will aspect into this scenario - i.e. Laura said that David must come with her of his own free will. But I think it can be worked in, if you have a sharp enough shoehorn.
(8 ) Is Laura really murderously psychopathic, on top of also being a genuine supernatural creature?
I don't think so. Granted, she killed Dr. Guthrie and knocked Elizabeth out of the picture. But if she's a genuine supernatural creature, why should she display compassion to creatures who are members of a lesser species? (Perhaps this is the time to mention that I have a new cat. We haven't yet worked out which of us is the member of the lesser species, mostly because she'd rather play with my shoelaces than discuss weighty philosophical issues.)
(9) Was there a sense in which Burke's girlfriend, pre-apartment-fire Laura, wasn't the same person as 1967 Laura?
The pre-sanitarium Laura was definitely different from the post-apartment-fire Laura. More than one person noticed it. I don't know about the interval between the sanitarium and the time spent in Phoenix. As for the reason for the difference, I'll have to do a lot more thinking before I reconcile it with the whole phoenix legend. Roger said that part of what put her in the sanitarium was feelings of guilt over the series of events that led to Burke going to prison. Or he said something like that. I think that when he said it, he seemed to believe it, but that doesn't mean it was true. At any rate, he didn't say anything about her having an unhealthy relationship with fire. He did, however, comment once on the strong, exclusionary bond that existed between Laura and David when they were in Augusta. Conceivably (I'm thinking about my answer to question #7 here) that bond led to some display of weirdness that couldn't be overlooked, but which Roger, in the good old Collins tradition of denial, forgot as soon as Laura was committed. As for why Laura let Roger yell at her - didn't Laura yell right back at him?
(10) Was Laura a supernatural creature in 1956, during the Roger/Burke/manslaughter thing?
Again, I've got to do a lot more thinking about this. It just doesn't make sense to me that she wasn't a supernatural creature at that point, or that she had forgotten her supernatural nature. There's got to be a better solution, but I haven't come up with it yet.
(11) Is 1967 Laura not connected to Ra at all?
I think that's correct. As for questions about 1897 Laura...I think the answer is no, but it's been three years since I last watched that storyline, so I don't care to say.