You cannot actually see the patio doors from the position where the group were sat in the Tapas, there is a hedge obscuring it.
The window shutters can be pushed up from outside but not stay up, they will fall again once you let them go. The McCcanns say they cannot rememer if the window was locked, meaning all you need to do to open it is put your hand on the pane and slide it across. The window is ample big enough to climb through alone, especially for a prolific burglar. More difficult to carry a child through it without help. Easy to leave no trace, especially if wearing gloves.
The car park was at a higher level than the apartments. The wall between it is fairly high which someone could easily duck behind to hide if needed but you can see part of the upper window from the car park, yes. The resort was very quiet at this time of year though so if there was a lookout, it is feasible.