1)
What is the meaning of "Hemiku Shichesu"?
1.
Radak: They were deeply wayward and corrupted their ways.
2.
Malbim: They go deeply to oppose Hashem's counsel, and corrupted their ways with Zenus.
2)
What is the comparison to "Yemei ha'Giv'ah"?
1.
Rashi #1: Like the days of Pilegesh b'Giv'ah, in Binyamin (Shoftim 19). Radak - they supported the depravity done.
2.
Rashi #2: This refers to Giv'as Sha'ul - they requested a king and rebelled against the Navi's words (Shmuel I, Perek 25).
3.
Malbim: Their deeds were like Sedom's - they were evil to Hashem, and also sinned Bein Adam l'Chavero. Therefore, "Yizkor Avonam" and they will be punished for it.