Delaying Melacha until after Maariv:
In addition to waiting until three small consecutive stars are visible and saying Havdalah in prayer or saying Baruch Hamavdil, every meticulous person is to delay doing any [Biblical] Melacha until the congregation has recited the Seder Kedusha [i.e. the entire paragraph of Veata Kadosh] after Maariv. Any person [even if he is not a Baal Nefesh] who does Melacha prior to this time will not see a good omen from the Melacha. [Thus, every man is to be careful in this matter. The Zohar is very stringent on this matter and writes that lighting a candle prior to this time causes the flames of Gihennom to be rekindled prematurely, and consequently leads him to be cursed by the souls in Gihennom, and be severely punished.]
Lighting a candle-Sake of Mitzvah: Even a candle should not be lit until this time, unless it is being used (for a Mitzvah purpose [or Kavod Hatzibur] such as) to light up the Shul, (in which case it may be lit after the recital of Barchu). [However, today the custom is no longer to delay until Barchu is recited, and rather one may do Melacha for the sake of a Mitzvah even prior to the recital of Barchu.]
Rabbinical Melacha prior to Seder Kedusha: One may move a candle and do [any Rabbinical] Melacha of the like prior to Seder Kedusha if the time of night for the leave of Shabbos has arrived and one said Baruch Hamavdil.
Summary [Includes Q&A]:
Men are not to perform any Melacha even after Shabbos concludes and one recites Baruch Hamavdil, until the congregation completes the recital of the paragraph of Viata Kadosh, unless one of the following apply:
1. The Melacha is only Rabbinically forbidden.
2. One is doing so for the sake of a Mitzvah.
If one is Davening at home, may he do Melacha as soon as he finishes his own Seder Kedusha in Maariv?
One who is Davening in private at home is to wait until the congregation which he Davens at finishes the Seder Kedusha.
May one smoke cigarettes before Maariv after saying Baruch Hamavdil?
It is improper to smoke cigarettes after Shabbos until the congregation has completed Maariv.
May women do Melacha prior to the conclusion of Maariv in Shul?
Ideally, even women are to be stringent not to do Biblical Melacha until after Seder Kedusha is recited in Shul. Practically, however, the widespread custom of women is to be lenient to begin doing Melacha once the proper time has arrived and having said Baruch Hamavdil.
If the electricity went out on Shabbos may it be turned on before Barchu of Maariv?
Yes. It may be turned on after reciting Baruch Hamavdil Bein Kodesh Lechol.
Is a mourner to delay lighting candles for the Amud until after he says Barchu?
There is no need to do so, as long as he says Baruch Hamavdil prior to lighting the candles. Nevertheless, the widespread custom is not to light candles by Maariv of Motzei Shabbos.
May a Shul begin setting up the accustomed video of the Rebbe after Shemoneh Esrei of Maariv but prior to Ata Kadosh being recited?
While there is room to argue that it may be done, practically, they are to delay doing so until after Ata Kadosh.
 Admur 299:19; M”A 299:17 in name of Darkei Moshe; Darkei Moshe 294 in name of Or Zarua Hilchos Motzei Shabbos 90 and Zohar; Zohar Bereishis 14b and Bamidbar; Ben Ish Chaiy Vayeitzei 2:24; M”B 299:40; Piskeiy Teshuvos 299:15
 Admur 293:1
 Admur 299:19 and 15-16
 Lit. Baal Nefesh
 See below in coming footnotes!
 See M”A 299:15 and Tur ibid in name of Yerushalmi and Ben Ish Chaiy ibid that one is to finish the entire Tefila
 Admur ibid; Pesachim 50b regarding Motzei Shabbos and Tur 299 end that it refers to before Seder Kedusha, prior to concluding the SederTefila
 See Q&A regarding women
 See Zohar ibid, quoted in M”A ibid, Darkei Moshe ibid and Piskeiy Teshuvos ibid
 See M”A 299:17; Kaf Hachaim 293:7 that according to the Zohar doing Melacha before Kedusha Desidra is repelling to Hashem.
 Admur ibid; M”A ibid; See Elya Raba 299:22; Raavan 385
 Zohar ibid, brought in M”A and Darkei Moshe ibid
 Admur in parentheses
 Derech Hachaim; M”B ibid
 Admur ibid, Parentheses are in original; Rama 299:10; Ketzos Hashulchan 95 footnote 6; However, before Barchu is recited it may not be lit.
Background: The source for this ruling is in the Rama 299:10; Ketzos Hashulchan 95 footnote 6 explains the reason for this ruling of the Rama is because in times of the Rama they would not say Baruch Hamavdil and hence Barchu would represent the leave of Shabbos. Thus, they would delay until then. Now although according to this reason one who says Baruch Hamavdil should not be limited to wait until Barchu, nevertheless we do not desire to swerve from this custom of waiting until after Barchu. [This ruling of Admur to wait until Barchu even after saying Baruch Hamavdil is an original ruling not brought in other Poskim, and hence perhaps for this reason it is written in parentheses.] Alternatively, Piskeiy Teshuvos 299 footnote 132 explains the reason is because according to the Rama the holiness of Shabbos extends until after the recitation of Barchu as is evident from the ruling that one is to lengthen Barchu [See Admur 293:4]. Thus, one is to delay Melacha until after Barchu. Alternatively, Kitzur Halachos p. 121 explains the reason for waiting until Barchu is because Admur rules one is not to do Melacha in front of the congregation prior to Shabbos being let out with the saying of Barchu. Since this ruling is a novelty that has not been brought in other Poskim, therefore he writes it in parentheses. [See there in length for the basis of this explanation]
 Ketzos Hashulchan in 95:2 completely omits the need to wait until Barchu, and in footnote 6 he explains it is no longer the custom to do so. So concludes also Piskeiy Teshuvos 299:17; Similarly, today the custom of all is to remove their shoes at home when Tishe Beav falls on Motzei Shabbos, and they do not delay to remove his shoes after Barchu, despite this being the clear ruling of the Rama in 553:2; To note that this was likewise the custom of the Rebbe to enter Shul for Maariv wearing non-leather shoes, despite the Rama’s ruling to delay until after Barchu. [See Hiskashrus 950]
 Admur ibid; M”A ibid
 Machatzis Hashekel end of 299, ruled also in Piskeiy Teshuvos 299:15
The reason: As some even allow Rabbinical Melacha to be done during Bein Hashmashos regularly, and certainly it may be done now before Seder Kedusha.
 Ketzos Hashulchan 95 footnote 5. Vetzaruch Iyun if majority of the city has completed Maariv if one is still to wait on his congregation. [ibid] See Piskeiy Teshuvos 299:15 which suggests based on the above Ketzos Hashulchan that one does not need to wait for his congregation if majority of the city already finished Maariv.
 Piskeiy Teshuvos 299 footnote 125 quotes this ruling in name of Kaf Hachaim 299:65, although in truth one who looks there sees he never discussed smoking before Maariv but rather before Havdalah. In any event the ruling stands true based on the ruling above.
 As it is best not to do Melacha until after Seder Dekedusha, as stated above.
 See M”A 299:15; Darkei Moshe 294; Tur 299; Yerushalmi Pesachim 4:1; Tashbeitz 88
 See Toras Shabbos 299:15, brought in Piskeiy Teshuvos 299:15
 See above in the Halacha and the footnotes there
 As a) It is for the need of a Mitzvah and Kavod Hatzibur and b) It does not involve Biblical Melacha.