When the MC wanted to save Inessa, he found a way to do it, so she could have done the same. When the MC showed up in the "human world" or whatever they called it, they talked about having a lot of angels looking for him, so I'm pretty sure they would be able to find one demon, and through him, the way to open a portal.
Also, the moment Raphael mentioned their mother would be there in a matter of minutes in that field, the older brother (who's supposedly stronger than the MC) almost shat his pants, took the even more powerful guardians and they all left with their tails between their legs... So she's someone to be feared.
The MC was supposed to die the day he escaped, but we know he was there for centuries, since he was a just child and he was praying every day to his parents and siblings who just ignored him... There's no way they can explain why they abandonded him. What would be their excuse? It was "written"? A caring mother will bring hell on earth in order to save her child. We've seen mothers moving cars in real life to save their children. If the dev was planning to have a redemption arc, he shouldn't have pushed their "shittiness" to the extremes. I really hope he won't go with something stupid like:
- Your dad didn't let us help you.
- Oh, that explains everything! I forgive you!
- Great, let's fuck!
We don't know yet a lot of the circumstances behind MC's exile, but for the sake of funsies let me try and defend Mom.
-There is an "apparent" balance (hence why Heaven wants the MC to take care of Hell) so Mom interfering and going to Hell to rescue MC might throw everything out of balance and screw MC even further.
-There could be an interference around MC (before his escape) in general, that would explain why Heaven or Mom or whatever can't hear his prayers.
-And lets say (and this one is my personal favorite but I doubt the dev will do this) that in fact "everything" was written, and MC for some reason introduced free-will into the equation, that would be cool and would certainly explain why Mom didn't intervene at first (because she would have been incapable)
-There is also the possibility that MC was declared dead, and not exiled or missing.
The thing that makes me want to believe Mom and why I'm willing to defend her (for now) is that for whatever reason she is in the Human world, either self-imposed exile, or there is some plan behind her actions.
Oh and also, at the moment we haven't seen Angels travel to hell, only MC but its due to his copy-cat powers and black magic. Also "shadow" magic ball stuff that Nyx did. We see them fly to heaven, but maybe they are literally incapable of traveling to Hell by conventional means, just like maybe denizens of Hell can't travel to Heaven willy-nilly.