This idea of not becoming a Buddha until all beings have emerged from samsara is in some sutras I've noticed.
It's really very weird. I usually hope when I see things like this that it is just some gross mistranslation.
I'm sure you could rationalize it with something like, "beings that don't have the karma to come in contact with a Buddha can come in contact with a Bodhisattva of yada yada level".
Close. I think the idea is that a Buddha like Sakyamuni has become nothing but Dharmakaya, which cannot be seen or heard. People need to be able to see and hear Dharma, otherwise they have no knowledge about it. So the pledge to not attain buddhahood is saying that they will continue to be accessible to sentient beings.
The goal of Mahāyāna Buddhism is non-abiding nirvana in which a buddha always acts on behalf of sentient beings, as opposed to the hīnayāna concept of buddhahood in which a buddha ceases his activity at parnirvana.
Maybe so, but ya still can't see