I could nitpick but truth is I cannot disagree with the bulk of your comments above. I can see how many would also find the film "distasteful and depressing" and be annoyed by the "crazy visual style". The description of the film as "fragmentary" is clearly deserved. Then again, my reaction to Tarnation differs significantly.

A 20-years-in-the-making autobiography in the form of an underground film, one that has absorbed a variety of influences from what was once called the "avant garde". Techniques that have infiltrated pop culture mostly via music videos. It's a video diary from someone who found an outlet of expression at an early age within the fringes of the art world. Mr. Caouette has chosen to present the material in raw form, with only intertitles to provide a bit of structure and chronology. Processing and interpretation of the material is almost completely the viewer's job.

This viewer found two basic organizing principles useful:
First, the film can be viewed as a son's attempt to depict his relationship to a mother who was in one or more ways, absent.
Second, the film can be interpreted as an illustration of how a person who is subjected to child abuse and family dysfunction, who appears to give in to self-destruction, can experience some degree of healing and rebirth. Can you see how it can be argued that Tarnation has a sort of "happy ending"? Taking into account what Mr. Caouette endured, I found it remarkable that he seems quite capable of developing supportive, long-term relationships, that his interactions with family members are free of recrimination and blame. (He does not "accuse" Adolph of abusing Renee. Caouette seems to be exploring an allegation made by one he knows to be unreliable as an informer. I did not sense any bitterness on his part when addressing Adolph on this subject.)

I understand how one would experience frustration at so much left unsaid, at leaving the theatre with too many questions. But I feel I know enough to feel endearment towards our tarnished angel and empathy towards his tragic mother.