So ?
Amy arrived and saw the window and shutter open from the front door side. She could have seen that from over the outer wall had she peered thru the waving branches of the trees.
When she entered the apartment she saw Kate and Gerry and this female friend (almost certainly Fiona). She could see them, but she could not see the twins, because Kate/Gerry had shut the door and closed the window and shutter at the same time, in order that they wouldn't wake up with all the noise. And have done that within seconds of Amy seeing the open window and raised shutter.. It only takes seconds, but longer (more seconds) to go around from the front to the back, then gather oneself for approaching Kate and Gerry.
This needs to be checked with the statements, but Amy might not have even gone into 5A seeing all three from the patio entrance? You cant just cherry pick bits like that Gunit, you have to think of the whole picture and what conceivably might have happened.
It is entirely plausible that Amy saw the window and shutter open and also saw the three, Kate, Gerry and ?Fiona.
At least you now accept that Amys statement was not "iffy" as was described it before.
She confirms that, on the night of the disappearance she was on duty and immediately went to the bedroom to see if the girl was hiding. She saw that the shutter was raised and that the window was partially open. It was then that she began to look in the wardrobes to see if the girl was hiding.
https://www.mccannpjfiles.co.uk/PJ/AMY-ELLEN-TIERNEY.htm 'At that point, Gerry, I don't think was in the apartment, it was mainly Kate. And Kate was just, huh, utter disbelief and I had disbelief, thinking she's got to be here, you know, what, how can this have happened. And by that point Kate was already saying that the, what she'd found when she'd gone back, which was that the, she'd found the window open and the shutter open and she was convinced at that point that somebody had taken, taken Madeleine and that's what she was telling me and I was like 'They can't have done. They can't have done this', you know. And I looked, I looked throughout the whole apartment and I looked in all the cupboards, under the drawers, under the beds, behind the curtains, everywhere, erm, just, you know, trying to, knowing it had already been done, but you just do. Erm, tut, I looked, when I went into the room that Madeleine was sleeping in, the room was dark, Madeleine, erm, Madeleine's bed was sort of folded back, the sheets, quite kind of neatly really, erm, Sean and Amelie were fast asleep in their cots, they didn't stir, you know, I was opening the cupboards in the room and moving around the room
https://www.mccannpjfiles.co.uk/PJ/FIONA-PAYNE-ROGATORY.htm