Who said the slaughter of the people of the book is halal? Most Shi'a fuqaha rule it as impermissible to eat either by fatwa or obligatory precaution.
Only meats slaughtered by Muslims should be eaten.
It is quoted in history that Imam Ali a.small never took war booty till prophet pbuhhp distributed it. The reason here would be more critical. Either that slave lady be much pious or was caught by the eyes of infamous persons. Otherwise, Imam wouldn't have done that. Another question is that taking as slave is not always in marriage. However, the marriage among masomeen with slave girls were not done without consent. The question that if Nikah was necessary is unclear because Quran says which you right hand possesses which means to me there should be legalitythat's what right hand means. But if hadith says that coming into possession is legality itself then it's valid as well. However, consent was always necessary and there are evidence. There is another thing, the marriages with slave girls are governed by same rules of marriages with few changes such as two divorce are for slave and three for independent. Also rules of law exempted slaves from full punishment, they were granted half punishment as that of independent.