[1] 691/10

[2] What type of Chumash is this Halacha referring to? Some Poskim [M”A 691/10; Bach] explain that this refers to a Pasul Megillah, such as a Megillah written on parchment and is sewed and rolls like a Sefer Torah, however has some invalidation, in which case according to some opinions [see Rama 143/4] one fulfills his obligation in a time of need. However by a printed Chumash one does not fulfill his obligation according to any opinion, [and hence there is no need to read from it]. [M”A ibid] However other Poskim [Mateh Yehuda brought in Kaf Hachaim 691/35] rule that it refers even to a Megillah written in a Chumash, as in a time of need one fulfills his obligation with it. Practically although we rule a printed Megillah is invalid, even in a time of need [Beis Oved 691/23], nevertheless one is to read from a Chumash if nothing else is available. [Beis Oved ibid; Mamar Mordechai 691/5; Kaf Hachaim 691/35; P”M 691 A”A 10; M”B 691/27] 

[3] M”A 691/10; Bach

The reason: According to some opinions [Rama 143/5] in a time of need one fulfills his obligation with an invalid Megillah. [ibid]

[4] Mamar Mordechai 691/5; Kaf Hachaim 691/35; P”M 691 A”A 10; M”B 691/27

The reason: Although one does not fulfill his obligation with a Chumash, even in a time of need, nevertheless this is done in order so the precept of reading the Megillah not be forgotten. [P”M and M”B ibid]