Does the discharge of a viscous, milk-like fluid from a pregnant woman's vagina nullify ablution? And what is the ruling on a man marrying, divorcing, and taking his wife back over the phone without granting her conjugal rights?

If the fluid is semen, then ghusl (definition) (ritual bath) is obligatory. If it is madhiy (pre-seminal fluid) or vaginal discharge, then it nullifies wudu (ablution). If there is doubt about the type of fluid discharged, one has the option to choose.

As for the husband marrying over the phone, if this marriage is deemed valid due to the fulfillment of its conditions and pillars, then any subsequent divorce and reconciliation within the iddah (definition) (waiting period) are valid. If it is not deemed valid, then the husband must separate from that woman. If she is already his wife, then he must be just in dividing his time between the two wives, unless the woman waives her right.