I posted it in the wrong topic.
From David's Twitter:
"Slaver" is "azzafrok"; slave is just "zafra".
And about why the double vowels are pronounced as 2 separeted vowels and not as a long one:
It's the same in Spanish. Spanish "creer" > Latin "credere" ("to believe", NdR). The "d" was lost.
Dothraki "khaleesi" > P.P. "khaleɣesi".
Dothraki "qazeroon" > P.P. "qazeroɣon".
So, it seems that the "ɣ" simply dropped, leaving the e-e/o-o pronounciation