Bard tries to convince the Armor that it will be following the letter of it's command, if not the intent, by opening the door for us so that we can pass by safely without causing any harm to the door. Probably offers the Armor friendship and freedom, along with better treatment than the BBEG. With expertise, surely not an impossible roll.

After that, when we make it to the witch, the bard makes the argument to her assuming that she is not outright suicidal/omnicidal that when the BBEG becomes a literal god that was even more evil than before it would be just as likely to hurt her and anything she cares about as much as anything else. Hopefully she sees, quite logically, that the BBEG's plan coming to fruition is as bad for her as anyone else and dishes on their weaknesses. We then use that info to fight the BBEG and disrupt the ritual.

Relies on two fairly high persuasion checks most likely, but with expertise, guidance, inspiration, maybe someone helping to give advantage, it's definitely not impossible.