Reviewing, I see that posting as a distraction from sickness or other issues can immortalize a temporary biliousness. My apologies.
Well from what we've seen the mother was pretty much a complete innocent in all this. It was Harry who appeared to shape the futures of Dexter and Deb the most, either through his law enforcement career or through the "code" that he taught Dexter.
In any case, I can't say I have much interest in exploring Dexter's past anymore anyway. By this point, the story should have been focused much more on whatever future he might have, and with his big showdown with the police. That's what we've all been looking forward to the last few years.
I can see the point, but the writers didn't agree with you. They're the ones who raised the issue of the past by writing in Charlotte Rampling. It was bad enough that we couldn't visualize the Morgan's family life. But when the surrogate Mom shows up, memories of the real Mom are going to surface.
In season four or
since season four?
The Lithgow Trinity killer was the highlight of the series, IMHO. Although perhaps I remember it poorly, was Rita unusually, over-the-top annoying in that season? Oh well, it all worked out in the end.

I thought Lithgow was a fascinating foil for Dex. Can't say that the seasons since then have been incredibly satisfying.
I guess if you never "bought" that Litgow was the monster he was, it wouldn't have worked. For me, he well surpassed whatever remaining typecast ideas I had about him being from either 3rd Rock or Harry and the Hendersons. I thought Lithgow channeled some complex performances out of The Trinity Killer. If I ever rewatch Dexter it'll be seasons 1-2 and 4. That's it.
Lithgow played a serial killer in Blowup and a lunatic in Raising Cain. There were some absurdities in the particular pathology he was supposed to manifest, such as the preposterous cycle four (not a Trinity) of killings each with its own insanely elaborate victimology and ritual. But Lithgow's excellent performance usually sold the character.
But the series is called Dexter. In season one, Dexter rejected serial killing with his brother as a way of life. In season two, Dexter rejected serial killing with a hot girlfriend and chose to surrender himself, until Lila provided an out for him. In season three, Dexter rejected serial killing with a friend. Part of the problem should be obvious by this point, namely, like most soaps the series was puppeting the hero through the same story over and over. Also, the quality of writing was steadily declining. Turning flashback Harry Morgan into a stupid cliché projection of Dexter's own personality was a key milestone marking the degeneration.
Now, as nicely sensational as Lithgow was, by season four, the story would have been, according to the formula, Dexter rejects a serial killer who has a family as a model. It was already way past time to move on to another story, as in Dexter decides to commit to Rita. Or, Dexter decides to quit killing. Really, if you were to treat Dexter's decisions in seasons one and two seriously, such a development would have been the logical, almost inevitable one.
Instead, they opted to tread water, playing around with foolish plots where Dexter was the vicarious agent of righteous retribution fantasies. Rita was hampering these fantasies, partly by an inconvenient presence making Dexter's activities seem even less plausible. But also because Rita's presence kept raising the issue of what Dexter was really going to decide to do: Get off on killing, or have a real life. Killing off Rita kept Dexter from making any choice at all. In a way, it broke the formula. But it did so by just decapitating the entire season, without replacing the old formula.
You know, the Dexter who was going to surrender rather than kill Doakes, the Dexter who chose Rita over Lila, just isn't the Dexter of the later seasons. In a way, it's strange that the show chose to disembowel itself this way. But if Dexter was really about a dream vigilante getting away with it, first, they should go to the trouble of making the plots halfway plausible. Second, Dexter should have dumped Rita for Lila.
Maybe the series would have been over? That is the best explanation of why the series went bad the way it did. But this problem was inherent to the open-ended serialization format. I simply cannot agree with people who say that there is no intrinsic difference in formats, it's all a matter of execution.
It's common to say the first two seasons were the best. It's also common to say the fourth season was the best. But the first two seasons and the fourth season undermine each other. And since they do, I do in fact include fourth season as the terminal season for quality writing in Dexter.
Doris Morgan has been pretty much a non-entity since the beginning of the series (the fact that I had to look up her first name says a lot). We know she died of cancer when Deb was sixteen, but since Dexter is ten years older than his sister, Doris must have been a part of his early adult life. It's sort of surprising, in retrospect, that the show didn't explore Dexter's relationship with his adopted mom a bit more.
It was especially surprising since an interest in Dexter's childhood was always a part of the series. I attribute it to Hollywood's pervasive problem with women. If you put Dexter to the Blechdel test: Doris Morgan, talking to Deborah about anything, not gonna happen. In the particular case of Dexter, Doris Morgan looking askance at the way Harry treated Dexter or trying to compensate for Harry's peculiar behavior, true to life as it might have been, would have jeopardized in show one of the founding elements in the show's mythology. To wit, that a sufficiently loving father can impose a code on a psychologically disturbed youth. There's a reason we never got a flashback to Harry explaining that Dexter could never sexually abuse the kills, or even masturbate on the scene.