1. Yes, it would be great to have a single word name, but the odds of finding a fitting (i.e. not random) name that works but not taken is very low. Adrift was my first choice back in 2016 but there's a space game with that name on Steam already. Other possible names were used as book titles and I don't want to waste time defending the title in court.
2. Yes, it would be great to have an awesome, evocative, and super original title but what's evocative and awesome for one person is utter shit for another. More 'mechanical' X of Y title tend to be less polarizing.
3. Pilgrims works because it fits the purpose (colonization), the religious nature (to establish a Christian colony), and it means people traveling to a sacred place (the promised land). The loss of the original purpose and the loss of faith adds an interesting subtext. Shortening it to just Pilgrims would be like shortening John Carter of Mars to just John Carter. Earth indicates that they left it (i.e. space colonization). We can't use Centauri as the ship hasn't landed yet. Not sure Pilgrims of the Void would be a better name, but we can consider it if we do another round of voting.