A. Although Nikah to your ex-husband will be valid after the second husband divorces you and you observe the period of Iddat; it is sinful to plan it (the Halaalah) this way.
Rasulullah (Sallallaahu Alayhi wa Sallam has cursed people who marry solely for the purpose of Halaalah. (Ibni Maajah V1 P622).
If she gets married a second time and coincidentally the second husband divorces her or he dies and thereafter she marries the first husband again, they will not fall under this curse mentioned in the Hadeeth.
Remember that only Nikah with the second purpose does not suffice. After he makes Nikah with you, he has to share your bed too, i.e. he has to have a sexual relationship with you. (Al Hindiyyah V1 P473).
Mufti Siraj adds:
However, if a third party arranged this Nikah to you, without you speaking to the man directly, then this will be allowed and will not fall under the curse mentioned in Hadeeth. This means that you go to a third person and ask him to arrange a partner for you but to inform the would-be husband the reason why this Nikah should take place. In this way you will have no negotiation with the second husband.
Also note that for the Halaalah to be valid and for you to be able to come back to your first husband, it is necessary that the second husband has sexual relations with you.
Moulana Yusuf Laher
Checked and approved by: Mufti Siraj Desai