Madeleine was last seen alive by someone other than the McCanns (staff at the Creche) at approx 5pm the day she disappeared. So unless the child in the creche wasn't really Maddie, we can assume that she was alive and well as of 5pm. From that point on:
IF Maddie disappeared between 5pm and 8:30 pm , it would seem logical that BOTH of the McCanns would have knowledge of what happened to her. In this time frame, either she 1) died as the result of an accident, 2) died as a result of murder by one or both of her parents (or someone that the parents were willing to cover for), or 3) was willingly given or sold to a third party by her parents with the intention of never getting her back.
~Help from LE and/or EMS was not solicited by the McCanns during this time period, so regardless of the reason for Maddie's disappearance, it would stand to reason that the McCanns found it necessary to plan a cover-up, ie an abduction. If Maddie had been given to a third party, it would not be necessary to worry about disposing of her body; if she had died somehow, it would certainly be necessary to dispose of her body before notifying LE that she is missing. Gerry or Kate would have had time to do that between 5pm and 8:30pm. They could have planned on going to dinner where both were seen together, providing the window of opportunity for a stranger to have come in and taken Maddie. Where it gets sticky for me is that I would think it very difficult for the McCanns to sit and dine with friends afterwards as though everything was peachy, but anything is possible. Someone upthread mentioned the risk of friends going to check on the children, but if Maddie was already disposed of elsewhere, it wouldn't have mattered... if one of the friends had gone to check on the kids and found Maddie missing, all the better for the McCanns! But it just so happened that nobody else noticed, thus forcing Kate to be the one to find her missing.
~One more cover-up scenario for the 5-8:30 time period: Maddie was dead when the McCanns went to dinner and they had not yet had a chance to dispose of her body; they placed her in bed as though she were asleep and went to dinner as planned, hoping for an opportunity to dispose of her later in the evening. I think this scenario is unlikely, as it would have been VERY risky that one of the friends, or staff even, would have entered the apt for one reason or another and noticed that she was not breathing and unresponsive. So I think this is a possibility but extremely unlikely.
IF Maddie disappeared between 8:30 and 10:00, we have the same possibilities as listed above in the 5-8:30 timeframe... BUT we also must add another fourth possibility for this timeframe only: a stranger came in and abducted Maddie.
~If Maddie had died accidentally in the 8:30-10:00, regardless of who witnessed it or discovered her body, when did Kate & Gerry have the time to discuss it, absorb the impact of it, plan a cover-up, all the while keeping thier composure? It seems unlikely to me.
~If Maddie had been murdered by someone known to her in the 8:30-10:00 timeframe, it could not have been Kate. It could have been Gerry and/or some of the Tapas 9. This falls into place with Kate finding Madeleine missing when she went to check on her; however, we still have the question as to how did the murderer dispose of Maddie so quickly and so well, all the while maintaining their composure? I think that this scenario is only possible if at least two individuals were involved, helping move the body around, etc. That would likely mean Gerry and accomplice(s).
~If Maddie had been murdered by a stranger in the apt and then removed from said apt in the 8:30-10:00 timeframe, one would imagine that more forensic evidence of this stranger would have been found. I think this scenario is possible but unlikely.
~It is possible that Maddie had been "abducted" WITH the knowledge and consent of one or both of her parents. It could have been planned well in advance. It is possible that one of the parents did not have knowledge of this. It would explain how the abductor felt comfortable taking Maddie that night despite several Tapas 9 coming & going in the vicinity checking on their kids, not to mention at least one of the couples apt was right next to the McCanns, and there were several others on holiday out & about.
~And then there is the complete stranger abduction. Yes, it could have happened. Unfortunately, the McCanns provided them with ample opportunity. The problem I have with this theory is the McCann's behavior afterwards, which suggests at least some involvement.
So, when I look at all of the possibilities, I think what makes the most sense and what most likely happened to Madeleine is either:
1) She died between 5-8:30pm, the parents both knew how, by whom, and why, and chose to cover it up. They removed her body from the apt BEFORE going to dinner. They then planned it all out to claim a stranger had abducted her.
or
2) She was abducted between 8:30-10pm by a stranger while the parents wined & dined at the Tapas.
All of the above rambling is my observations and opinions only!!!