This is my first post after reading extensively and mulling over this case intermittently for the past several years, so please do not savage me.
I am not advocating such a theory although I can fairly easily posit at least two scenarios which would only involve one parent, not both. It is extremely curious to me that those who believe the McCanns to be involved always assume both are, rather than conceding the possibility only one parent is culpable and the other parent was, and presumably remains, completely unaware.
A theory of sole parent culpability seems to me to be no more or less plausible than all other theories and has the distinct advantage of explaining why the McCanns have sustained their efforts at publicity and police review over the years. If only one parent was involved, these efforts can be interpreted quite differently--not evidence of innocence of both parents (as many here argue) but only of one of them. For the other culpable parent, these efforts are not a counterintuitive charade to convince the public and law enforcement but more fundamentally a necessary façade kept up for their innocent spouse, who genuinely wants the search efforts to continue.
If such a theory of sole parent involvement has been developed or discussed, can someone please refer me to it? Again, I stress that I don't necessarily believe this but nor do I discount it. I have some interesting thoughts along these lines that would include a woke and wandered scenario and also account for the Smithman sighting. However, there is simply not enough evidence available to rule anything out, leaving only a range of more or less implausible --but not impossible-- explanations. All in my opinion, et cetera.