If you kill Gwyndolin, the Darkmoon Blade covenant will disappear from the list at the bonefire, thus not allowing you to join it back. You must go to NG+.

Are you kidding? Gravelord's Sword wins the game.

Nope. Cant join the covenant if the leader is dead.

Question, on Dark Souls Remastered, if I join the Blades of the Darkmoon for the Darkmoon Blade miracle, then kill Gwyndolin for the Sunlight Blade miracle, then try to join the covenant again through the bonfire, will I be able to rejoin to be able to use both buffs?

If they got rid of achievements tied to leveling up covenants I would agree with you but if the achievements stay I cannot agree with the punishment

you can only join 1 covenant per character

it wont reset but your offerings will be halved to 3 medals

imagine a vinheim spy covenant that allowed you to basically become corvo from dishonored

the highest rank you can get in any covenant (except certain co-op covenants) is +3

their all viable in offline mode its jsut harder to get the items to rank up in them but there is a place to farm every one of them (E.G. basilisks drop eyes of death and the harpies in ariamis drop souveneirs of reprisal)