A point to bear in mind is that the PT statements are not verbatim, they are only a summary, and there is room for confusion in the translation process, the general stress...
On the front door discrepancy, to me it seems like something got confused.
Gerry was one of the first (if not the first) to be interviewed. Wouldn't the officer have asked for a general description of the apartment to get a general understanding? If so, it's not recorded as such. There are no real details of the general routine in this interview, but we don't know if there had been a discussion for the officer to get a general idea or not. The officer and/or the translator could have been scribbling notes and could have got mixed up.
It makes no sense to me for Gerry to say that he and Kate had taken the long way around via the front door using a key, whilst in the same breath stating that Matt had gone to check via the unlocked veranda door. Why didn't the officer ask the obvious question at the time to clarify?
In this way, at about 21.05 the witness came to the Club, entered the room using his respective key, the door being locked, went to his children's bedroom and checked that the twins were fine, as was Madeleine. "He then went to the WC" where he remained for a few moments, left, and bumped into a person he had played tennis with and who had a child's push chair, he was also British, he had a short conversation with him, "returning after that to the restaurant." At about 21.30 his friend Matt (member of the group) went to the apartment, where his children were and on his way went to the witness' apartment, entering by means of a glass sliding door that was always unlocked and was located laterally to the building. He entered the bedroom, he observed the twins and he did not even notice whether Madeleine was there."