Archdukesol
Since Freyja´s father is named Njord she should be Njorddottir. I know that marvel fucked it up spectacularly since they fused Frigg and Freya into one and on top of that fucked whole thing over. Freyr is Freyja´s brother not father. Its quite painful to read Freyrdottir which basically means daughter of Freyr and immediately after read that she is the daughter of Njord :D So choose just one of them please :D
In marvel death is not a definite thing freyja could reincarnated into another body and with their new bond he could probably feel a pull on his senses as he gets stronger. Black widow is best as him meeting her and unlocking her divinity could heal her from sterilization, power her up to top powerhouse levels, and save her from self sacrifice. The pain from his loss could drive him into training and a time skip eventually.
Death is literally a entity in marvel and several people come back from her grasp in marvel. And black widow was a favorite example. And the vow could be interpreted a few ways. It could be loyalty even in death. To not let death keep them apart. It doesn't have to mean to die with her if she dies. What about revenge? Couldn't he make a deal or bargain with death for her to return to the living or she could be the one dealing. Maybe neither of them. The mother maybe?