The Galactica travelled back in time at some point after the original series and before Galactica 1980
The fact that the original series ended with the Galactica seeing footage from 1969, clearly placed it some point after 1969, whereas 30 years had apparently passed before 1980. This creates an obvious plothole given 1980 is not 30 years after 1969 (trust me, I did really well in maths at school). However, given the casual and easy nature of time travel shown in Galactica 1980, it's possible that at some point the entire fleet (undoubtedly with the help of Doctor Zee) took a leap back in time, possibly to avoid the Cylons (knowing they weren't in a particular part of space a few decades ago). Alternatively to the above, the moon landing footage was transmitted by time travellers
It's possible there was a timeline we didn't get to see where the Galactica never received a transmission from Earth and took generations longer to get here. Renegade members of the Galactica's crew then decided to travel back in time in Earth's past (some time before the 1950s) and to transmit a message that the Galactica could follow to get them here much sooner. They decided to give them knowledge of Earth, its history, important events, and complete details of the Earth's location (though they still had to make use of Earth technology at the time for the broadcast). Of course, there were those on the ship who didn't support the plan and who stopped them, meaning they couldn't broadcast for long. The message degraded over time anyway, hence explaining why only limited footage survived to reach the Galactica but it was enough to change history and bring them to earth sooner. Apollo and Sheba are not dead. They became Beings of Light
Had the show not been cancelled, this would have been revealed as Starbuck's fate as well, since he did not return with Angela and the baby who would become Dr. Zee. Adama knows this because at some point, he visited the Ship of Lights but was allowed to retain his memories of his encounter with the Seraphs. The condition was that he would maintain a code of silence, which Adama has no problem with given that he believes that these beings of light are actually represent a higher level of God with the Lords of Kobol directly under them. This would also explain why he easily acquiesces to Dr. Zee given Zee's origin. Apollo, Starbuck, and Sheba faced down Count Iblis and won, so this makes them primary candidates for more special missions from John, the same Seraph who recruited Apollo for his mission on Terra. It also makes them perfect to challenge Iblis in his other endeavors, his alleged "forfeit" the Seraphs mentioned, being only a temporary setback. Apollo, Starbuck, and Sheba, in their new form as beings of light, are instrumental in keeping Iblis too busy to either threaten the Galactica again or influence the Cylons any more than he already has. Adama, keeping his word to tell no one, must continue to let Troy believe that Apollo is dead but being a man of faith, can wait for the day when the truth can be revealed.