So I can't speak for all Dortmund fans and as with everything there will be a big spectrum of reactions to this transfer. There are those that never wanted to have him back, those that are really happy about having a great player back in the team and those inbetween, that are just a bit torn, like me. I am sure there will be boos for him in his first couple of home games but that is to be expected. I don't think the reactions will be as harsh as you are describing.
I don't think your wife comparison holds up, this is still a job and maybe I approach the subject a bit more pragmatically than the average Sporting fan. But then again, sometimes relationships end and it isn't even unheard of that couples get back together after a break-up
What I strongly disagree about is you saying that Hummels failed at Bayern. I really don't think that is the case. He may not always have been perfect and had some weaker spells but overall he is still a very strong defender and showed this at Bayern. Especially the second half of last season showed once again what he is still capable of. So Dortmund is not buying him to do him a favor, they want him to play an important role on the pitch.
So what about my two cents? I am really a bit torn. I love Hummels as a player and he was one of my favourites in the Klopp era. The first name on my jersey since Dede. He could have been a Dortmund legend, the man to lead them to two Bundesliga titles and maybe even more had he stayed. He had left Bayern at a young age because they didn't rate him enough and he had claimed not wanting to go back during his stay in Dortmund. That is why I was shocked, when he left. Not because of the way he left or why but just because I did not see it coming at all. He had presented himself as Mr. Dortmund and then he left. I didn't hold grudges, I was just sad, that he was gone.
He can never reach that level of acceptance or adoration again. He will never be the icon that he could have been. Now he can merely be a really good player at my club. One that I don't really blame for the move he made and one that - should he be able to recreate his performances - I will love to see on the pitch in a Dortmund shirt again.