Removing a Sefer Torah and reading from it not for the sake of Kerias Hatorah:

It is forbidden to remove a Torah scroll for no reason or to read from it when not necessary.[1] Thus, an individual who did not hear the Torah reading, is not to take out the Torah scroll to read from it the Torah portion of that day. However, it is permitted for a group to take out the Sefer Torah and read the Torah portion from it without its blessings even if a Minyan is not available. Thus, if a congregation could not gather a Minyan for Kerias Hatorah, they may nevertheless remove the Sefer Torah and read the entire portion consecutively without its blessings or Aliyos.[2]  [However, even by a group, it is forbidden to read from a Torah scroll if he does not know where to stop by each verse.[3]]

Reading Shnayim Mikra from a Sefer Torah:[4] If one knows by heart the tunes [which mark the end of each verse[5]] in the Torah reading, it is proper for him to read Shnayim Mikra from a Kosher Torah scroll on each and every Erev Shabbos.[6] [This, however, is not the Chabad custom, nor the custom of many other Gedolei Yisrael, as by doing so one is unable to read the Targum after each verse.[7] Hence we read it from printed Chumashim which contain Targum.[8] One who does not know the Taamim by heart, is according to all not to read from the Torah.[9]]

Reading the Nesi’im from a Sefer Torah:[10] It is not the Chabad custom to read the portion of the Nesi’im from the Torah scroll.[11] However, some[12] are accustomed to do so, and those that do so have upon whom to rely.[13] However, those who follow this custom, should not read the portion in the Torah after the Torah reading on Shabbos, but rather after Musaf.[14]

[1] See Shach Yoreh Deah 270:5; Taz Y.D. 270:4; Derisha 270; Maharsham 1:175; Meishiv Davar 16; Minchas Yitzchak 19 “Appears like removing the Torah scroll without need”; Shaareiy Halacha Uminhag 2 pages 77-79 regarding the Chabad custom of not reading the Nassi or Shnayim Mikra from a Torah scroll

The reason: As today we no longer learn from a Sefer Torah, and it is even forbidden to do so, as it is belittling to the Sefer. Thus, an individual is to read from a Chumash. [Derisha ibid, brought in Taz ibid and Shach ibid; Likkutei Sichos 23 ibid]  

Other opinions: Some Poskim rule that there is no prohibition to remove a Sefer Torah to read from it even as an individual. [Nemukei Orach Chayim 669:2 towards end “It is incorrect to say that there is a prohibition to read from a Sefer Torah, as where is this written at all, and if someone wants to learn from a Torah scroll in public, as they say of the Gr”a…”; Chazon Ovadia Pesach; So rule regarding Shnayim Mikra: Admur 285:4 Arizal Shaar Hakavanos; Taz 285:1; So rule regarding the Nassi: Nitei Gavriel Pesach 1; Piskeiy Teshuvos 429 that so was custom of Talmidei Habesht]

[2] Elya Raba 143:2; Makor Chaim 143 of Chavos Yair; Shaareiy Efraim 7:38 “A town that does not have a Minyan and the group desires to read the Sedra in the Torah scroll without a blessing, may do so”; Piskeiy Teshuvos 143:1; Mayanei Hayeshua 9; See Nesivim Bisidei Hashlichus 2:4 footnote 10; So rule regarding Parshas Zachar: M”B 685:17; Kaf Hachaim 685:35 based on Rama 685:7; Kinyan Torah 7:55; See Shraga Hameir 6:116; Binyan Shlomo 54

[3] Admur 494:11, and so is implied from Admur 285:4 which writes “Piskeiy Taamim”. Meaning that he knows where each verse stops. [Ketzos Hashulchan 72 footnote 1, derived from Admur’s additional word “Piskeiy Taamim”]

The reason: The reason one must know the end of each verse by heart as a condition to read from the Torah is because it is forbidden for one to stop in middle of a verse that Moshe did not stop by [494:11], and if one does not know the end of each verse certainly he will end up stopping in middle of some of the verses. Vetzaruch Iyun for why this condition was not mentioned by any of the Poskim who deal with the question of reading from the Torah.

[4] Admur 285:4; No leniency is however to be learned from this law regarding reading from a Sefer Torah without due reason. See Shach Yoreh Deah 270:5; Nemukei Orach Chayim 669:2 towards end; Shut Marsham 175

[5] Meaning that he knows by which word each verse ends. [Ketzos Hashulchan 72 footnote 1, derived from Admur’s additional word “Piskeiy Taamim”] The reason for why one must know the end of each verse by heart as a condition to read from the Torah is because it is forbidden for one to stop in middle of a verse that Moshe did not stop by [494:11], and if one does not know the end of each verse certainly he will end up stopping in middle of some of the verses.

[6] This was the custom of the Arizal [Shaar Hakavanos] and of the Taz [Taz 285:1]. The Taz [ibid] rules that each person should read the Mikra at least one time from a Kosher Sefer Torah, as from a mere Chumash one does not properly fulfill his obligation.

[7] The Arizal however would have a student read him the Targum and he would then repeat it after them. [ibid]

[8] Hisvadyus 1988 Vol. 2 p. 167; Lekutei Sichos 24 p. 214 footnote 59; letters printed in Shaareiy Halacha Uminhag 2:189

[9] As he will not know by which word to end each verse. See previous footnotes

^[10] Shaareiy Halacha Uminhag 2 pages 77-79

^[11] As was seen by the Frierdiker Rebbe, and above by his father. Similarly, they were not stringent to read Shnayim Mikra from a sefer Torah [Rebbe ibid] Some Poskim rule one is not allowed to remove the Sefer Torah for this purpose, as it is forbidden to remove a Sefer Torah for no reason. [Maharsham 1:175; Meishiv Davar 16]

^[12] See Nitei Gavriel Pesach 1; Piskeiy Teshuvos ibid that so was custom of Talmidei Habesht

^[13] see Chazon Ovadia Pesach who explains in length that there is no prohibition involved here of “moving a sefer Torah for no reason”

[14] Rebbe ibid; See Piskeiy Teshuvos ibid