Op, I've been saving this post of yours in my drafts for *checks* months... because I've had thoughts, but didn't know how to express them, or even, if I should. I am a little worried that I'll be derailing your very real frustrations with the dead wife/fridged character trope, but I do want to add a slightly different perspective on what might have been the original intention of our lack of info on Maria.
In SA2, in the original game, I think not getting a chance to know Maria is by design, and subsequently, part of the tragedy.
Whatever relationship Shadow did or did not have with Maria (which, I would argued, it was implied in the game before they reconnected the timeline that Shadow didn't actually get a real chance to know her and that I very much explore here if you are interested.), his recollection of her and that relationship was destroyed when Gerald Robotnik decided that his grief and rage were more important than anything else, including his grandaughter.
Japanese: "Based on the origional data, I have finally completed Shadow. I also completely control his memories. I leave everything to Shadow."
English: "Based on my original projections I was able to complete my project, Shadow. I designed his mind to be perfect, pure. I will leave everything to him."
Shadow's brain was scrubbed and rewired to only remember most clearly Maria's death and Gerald's plans for destruction. Any other memories of Maria as she was died with Gerald and all the others aboard the ARK. Not even Ivo really knows her, and that might be because he was small when his cousin went to space, or because Maria herself was small and thus hadn't developed a full personality yet. The people that got to know her the fullest, as she was when she was most herself, are gone.
We are left with nothing but pieces to pick up and shreds of inference.
Much like Shadow himself.
We, like him, are suppose to have complicated thoughts and feelings for this girl we don't know, because he has complicated thoughts and feelings for this girl he doesn't really know.
And he loves her- as much as he can- as much as he knows how- as much as his damaged brain can let him. If nothing else, he loves the idea of her- the concept of that love and freedom and hope and joy and care and sacrifice. He must have loved her for more than just the ideas she represents to him once, right? She must have loved him, too. She sacrificed herself for him.
The clearest memory he has of her, across every game, is watching her die. Maybe the specifics change - the location, her style of dress, the way she did her hair- maybe they change, but still, the clearest memory he has is watching her die, and him not being able to do a damn thing about it.
Not her favorite food, not her favorite stories, not even her favorite colour.
Just her death. Just him not being able to do anything. Just the gap of where his genuine love for her was taken out of him and replaced with something horrid.
That is a very, very specific sort of grief, and I wish Sega explored it more.