You write in PbP: " Since these six can marry others (but not brothers of the Yavam), their 'Tzaros' are permitted (they are not really Tzaros, for the Ervah is not truly married"
Why do you write this? It seems clear that the ervah is married to a nachri and when he dies the tzarah can marry one of the ervah's asurim
Tuvya Marcus , Jerusalem Israel
If the ervah is married to a nachri, she would not be called Ervah!
Rather, the Ervah 'married' her relative (he gave to her a ring under a Chupah, read the Kesuvah, blessed Sheva Berachos (l'Vatalah)
and invited 500 guests to celebrate the 'Chasunah', and they are living together like man and wife).
The Torah does not consider this a marriage; it is mere Zenus; the man's only wife/wives is/are the Tzrah/Tzaros, so she/they are not Tzaras/Tzaros Ervah.
Pesach Feldman