Repeating Shema at Night

1. The time to say Kiryat Shema at night is Tzet HaChochavim (3 small stars across the sky or 72 minutes after Shekiah). If the minyan was Davening earlier one can daven with them that’s if it’s after Shekiah (or it’s after Plag Mincha and one already said Mincha before Plag Mincha) and then repeat Shema after Tzet HaChochavim without the Brachot.
2. When repeating Shema, it’s sufficient to say the first 2 paragraphs but a Yireh Shamayim should say all 3.
3. One should not rely on the Kiryat Shema Al HaMitah for several reasons including a) the Ashkenazic minhag is not to say all 3 paragraphs and b) a person doesn’t have kavana to fulfill Shema and also to accept the yoke of heaven when saying it. If one is insistent on using Kiryat Shema Al HaMitah as the repition one should be careful to say all 3 paragraphs with kavana.
4. It’s proper to say Kiryat Shema within the first half hour after Tzet HaKochavim because of Zerizin Makdimin LeMitzvot (alacrity for מצות). If one didn’t say it then, make sure to complete it by Chatzot (midnight) and after the fact, one can still say it until Olot HaShachar. If one was in extenuating circumstances one may say it until Netz HaChama.