I am going to chalk up the whole bizarre Yzak being Deakka's little bitch and switching sides so quickly to the fact that the show is boiling down to its end and they just don't have enough time to treat all the characters properly. Naturally I blame Fukuda and all the others for spending so much time on recaps and flashbacks and failing to have any new development with the old SEED characters. Just about every single person from the SEED series is back in the EXACT SAME SPOT as in episode 49 of SEED.

Mwu is still blocking the AA from getting killed (but an MS being able to withstand a shot a battleship can't?)
Lacus is still all pouty and sad. Waldfeld still doesn't have enough lines. Maryu is all dreamy about Mwu/Neo despite being amidst a warzone.
Deakka is STILL changing sides. Yzak is following his lead, because Deakka is his yaoi uke. (jokes, jokes)
Athrun is still deciding his actions for the most part of the war are wrong and that he should follow Kira's lead.
Then we have Rey = Rau and Talia Gladys sorta = Nataru, both sticking to their orders but having major doubts about fighting the AA.