Yes what you say makes sense.
BTW at about 11pm on May 4th when the dog's first response (starting outside) was "headed to the door" (of 5A), a straight interpretation is: the target is inside the apartment at this time.
And they did not open the door? The dog did not insist on his find?
IMO what you are saying is a big big speculation.
1.First you speculate that the scent on the source given to the dogs is not Madeleine's.
2. Second you speculate it is Fiona's scent on the source given to the dogs.
3. Third you speculate that the dogs did find Fiona Payne but she did not open the door although she was inside to let the dogs find her.
After all of these speculations which you take as FACTS. why then the dogs went to the car park?
This is how the internet myths are made up. People start believing that these are facts. Madeleine's scent absent from the towel is not a fact. We don't even know if this is some kind of a misunderstanding. Maybe the towel is actually Madeleine's blanket. How do YOU know it is not?
I can speculate it is. Same as you speculated these are two different things.