The gemara says that although we allow a breach in the side wall (or even front wall) of a mavuy to be as large as 10 amos without having to fix it, it must be in the actual wall, not in the corner. The language that the gemara uses is פתחא בקרן זוית לא עבדי אינשי. It isn't normal for one to make an entry way in a corner, therefore we don't automatically consider an entry way in the corner to be a pe'sach just because it is 10 amos or less. However, Tosafos says that if one constructs a Tzuras HaPesach which indicates that he does intend to use it as a pe'sach, that is sufficient. Tosafos holds that we don't automatically validate the type of breach that catty corners a square area, but a tzuras hapesach is a valid indication that one intends to use it as a pe'sach. The Rashba, although he agrees with Tosafos, cites that both the Rif in the name of Rabbeinu Hai and the Rambam disagree. They maintain that even a tzuras hapesach would not work if it is in a corner. Furthermore, the Rambam (Hil. Shabbos 16:20) holds that even if it doesn't quite catty corner the square area on the diagonal, rather it is a pe'sach that is made near the corner (i.e. the pesach is made in the North-West corner but is fully on the western side without eating up any of the northern wall), it is considered a פתח בקרן זוית and would not be valid, even with a tzuras hapesach. The halacha is like Tosafos, not like the Rambam. Another point is that when the breach is in the front of a mavuy, such as picture 1 below, and people use that opening to enter and exit, it is considered בקעי בה רבים and therefore must be less than 4 tefachim wide. The M.B. 365:10 discusses that idea that we may need to be machmir for city eriuvin that they have the stringencies of a מבוי. Therefore, picture 1 would be a problem even if we don't consider it to be a פתח בקרן זוית.