All you have to do, ferryman, to judge the professionalism of one of the nannies and her supervisor is to look at the Lobster Minis sign-in sheets. The afternoon of 30th April was particularly bad. Not being able to demonstrate who's there and who isn't is the height of bad practice. Given the childcare standards of the McCanns I wouldn't be at all surprised to find they think those of Mark Warner are acceptable.

I'm not sure you can rely on the afternoon entries as evidence of poor record keeping. What I am seeing is a particularly poorly scanned document. There appear to be multiple entries that have not been picked up, and even some of the box grid has almost disappeared from sight.
Just thinking out loud at the moment. Most forms tell you to use a dark ink, to get it to scan or copy well. MW crèche would not have needed a copy, so perhaps they were less fussy about this. Maybe, or maybe not.
What can be picked up from this page is interesting.
The idea that a substitute for Madeleine was being used strikes me as ludicrous, but let's chalk it off on the evidence.
M was in the same group as Ella O'Brien, as the record shows.
Therefore to use a substitute - a substitute has to first be arranged, then disappear. This requires a supplier is in on it. If the M child changes during the week, it requires that Cat B is in on it (but not if the child substitutes for M from Sun to Thu). It requires the substitute child is able to respond to Madeleine or Maddie, and keeps her own name secret. On this particular record, it requires that Ella keeps her gob shut about the fact that the substitute is not M.
It is difficult to discern the afternoon time out for M. Further, I haven't looked to see if there is a record for that day for the twins. But on any day the twins and the substitute took high tea together, the twins had to react to this substitute as if she were M.
I haven't checked the records or statements to see if any of the T7 attended a high tea. If so, they are in on it. Either the substitute was at high tea, which they definitely should have noticed, or the substitute wasn't, in which case they should have wondered why the twins were not accompanied by their sister.
Finally, there is the simple issue of looking after the substitute outside of crèche times. Breakfast, lunch, evenings.
To make the substitute theory fly, you need a massive conspiracy plus some cracking child actors.
What is the probability? 0%, IMO.
Unless I am mistaken, that cuts it down to whether the crèche records were being fiddled or not. Even if a substitute was used for a very short period e.g. just Thu afternoon, then Cat B would have to part of the plot. You don't look after a child from Sun to Thu morning, and not notice a sub has been popped in for one session.
So either M was there, as per crèche records, or the records are false. Sloppiness does not enter the equation.