Was Croix and his party the only ones to enter Frelia's Binary Field?
They aren't Diving into Frelia: Shurelia has free access to the Binary Field due to being the Tower Administrator, so she was merely playing along with Mir's game. But as for Aurica and Misha, they aren't actually there: what you saw there are replicas of them created from data copied from the Beta-6D SH Server where their Cosmosphere are located (according to the Garden of the Goddesses Visual Book).

Also, their proper names are Miu and Leila (the latter being another reference to AT1's Binary Field), so NISA failed once again with the names.