If the only available Challah I have for Lechem Mishneh is frozen solid - may it be used ?
Sh'miras Shabas K'hilchasah 55:12 says frozen bread is okay, citing B'tzel Hachochma (volume 3, 110:4) and Riv'vos Efrayim (volume 2, 115:2). (They allow it because one can extend his meal until the bread is edible; SSK notes, though, that he heard from Rabbi Sh'lomo Zalman Auerbach that it can't be used unless one plans to extend his meal that long, in which case the bread is "כרותח שצריכים להמתין עד שיצטנן" [which I don't understand].)
Rabbi Shmuel Kamenetsky in his Sefer Kovetz Halachos writes that one may use a frozen Challa for Lechem Mishna. Furthermore he writes one may use a Challa that is not Pas Yisrael for the second Challa even if he is Makpid on Pas Yisrael. (Do not have the Sefer in front of me now. I will add in the exact location when I have the Sefer)
Tzitz Eliezer 14:40 also permits using a frozen Challa for Lechem Mishna and a Challa that is not Pas Yisrael.