Once the blessing has been recited it is forbidden to speak from that point and on until the reading is completed [and the final blessing is recited]. This applies for both the reader and the listener. This applies even towards words of Torah and even towards matters that relate to the reading, but are not a necessity to be said.
One who talked between the blessing and the reading: If one talked [of unrelated subjects] after the blessing was recited, before the start of the reading, the blessing has been forfeited [and hence must be repeated]. This applies both for the reader and the listener.
One who talked during the reading: If the Chazan talked during the reading he may continue from where he left off as explained in the previous Halacha. However if the listener talked while the Chazan was reading then he does not fulfill his obligation and must repeat the reading [without a blessing, from the place he that he did not hear due to his talking].
One who talked between the reading and the final blessing: If the reader talked [of unrelated subjects] prior to reciting the after blessing of Harav Es Riveinu it requires further analysis as to whether he may still recite the blessing. Some Poskim conclude that the blessing may be said so long as one did not remove his mind from the reading.
What is one to do if in middle of reading Megillah he realized that he did not yet read Shema and by the time the reading is over the time of Shema will have elapsed?
He must stop and read Shema prior to the time elapsing. Practically, if possible, one should wait until Haman is sounded and say the first verse of Shema while everyone is making noise to blot out the name of Haman. If this is not possible one is to say the first verse of Shema in middle of the reading, and quickly read inside and catch up to the reader. See Halacha 5B in Q&A for the full details of this subject!
What if in the above case the Baal Korei realizes that he did not read Shema?
He must stop the reading and read Shema prior to the time elapsing. Practically, if possible, he should wait until Haman is sounded and say the first verse of Shema while everyone is making noise to blot out the name of Haman.
See Halacha 5B in Q&A for the full details of this subject!
Sheim Reshaim Yirkav:
In Rogatchev there was a Chassid named Yossele Tzeitlin which was a follower of the Alter Rebbe and served as the Megillah reader in his town. In middle of the reading, after reading the verse “To whom does the king desire to honor more than I” he suddenly stopped and began cursing and blaspheming Haman. The Rebbe was very amused by this act of the Chassid and hence gave him a gold coin donation. Other wealthy Chassidim followed in turn and hence the Megillah reader became wealthy. This fulfilled a blessing of wealth that this Chassid received from the Alter Rebbe.
 Rama 690/5; Michaber 692/2; Taz 692/2 explains that the ruling of the Rama ibid refers to talking during the reading while the ruling of Michaber ibid refers to talking between the blessing and the reading.
 M”B 692/9
The reason: One may not speak after the blessing until the beginning of the reading in order not to make an interval between the blessing and its Mitzvah. As well, one must refrain from talking during the reading since a) It is forbidden to make an interval until the Mitzvah is completed [as explained in previous Halacha regarding the Chazan] and even more so regarding the listener b) If even one word is not heard according to many Poskim one has not fulfilled his obligation. [M”B ibid; M”B 690/19]
 Rama ibid; Taz 692/2; M”B 690/19; 692/9; Shaar Hatziyon 692/12
 M”B 692/9
 Kaf Hachaim 692/21; Admur 167/9 [regarding Hamotzi]; 432/7 regarding [Bedikas Chameitz]
 M”B 692/2;
 Kaf Hachaim 692/21; Admur 167/9 [regarding Hamotzi]; 432/7 [regarding Bedikas Chameitz]
 Levush brought in Kaf Hachaim 692/21
 Nevertheless if the listener spoke in between, seemingly he may not say the blessing as he will lose out on the reading while he is reciting the blessing, and according to many Poskim he must hear every word of the reading. Regarding making a blessing after having begun the reading, and until when a blessing may be recited-see Halacha 10 in Q&A!
 Beis Yosef 692; M”A 690/7; Levush brought in Kaf Hachaim 692/22; 690/36; M”B 690/19; 692/9; Chayeh Adam 154/18.
 In M”B 690/19 he writes “according to some Poskim”; In 690/5 he writes according to majority of Poskim; In 692/9 he plainly writes one is not Yotzei.
 See Halacha 13!
 Shaar Hatziyon 692/12
 However regarding the listener, since he is anyways Yotzei with the readers blessing, there is no real question involved in whether he may say his own blessing, unless he did not hear the blessing of the reader, or is accustomed to say his own blessing. See coming footnotes. Nevertheless even the listeners must initially be careful not to talk until after the blessing, as explained above.
 According to the Tur since the blessing is said in relation to the reading, perhaps if one talked in between he may no longer say the blessing. However according to the Baal Haitur that the blessing is not related so much to the reading, certainly the blessing may still be said. [Shaar Hatziyon ibid] Seemingly in such a case one of the listeners should say the after blessing and fulfill the obligation for the reader.
 Birchas Habayis 55/19; Regarding those listeners that say their own blessing of Harav Es Riveinu: Piskeiy Teshuvos 692 footnote 9 concludes that if the listener talked in between then he should be Yotzei with the blessing of the Chazan. If however the Chazan already began saying the blessing [and one did not hear it] then he may say the blessing himself.
 Brought in M”A 690/22
 Some [Shaar Hatziyon 690/57; Kaf Hachaim 690/111] negate this custom as one may come to miss a word from the reading. Nevertheless it is not considered an interval being that these words relate to the Megillah. [Shaar Hatziyon ibid based on 690/13] However others suggest that perhaps from an initial perspective it is considered an interval and hence is not to be done. [Kaf Hachaim ibid]
 Otzer Minhagei Chabad p. 267