Well, here are a few notes from off the top of my head.  I recall, when
doing my dissertation, that I thought much too much had been made of
late-life crises in Twain's life as influences on his world view.  In that
fool thing, my dissertation I mean, I tried to show that much of Twain's
so-called late-life disgust with religion and such were actually more mature
musings on ideas he'd had since childhood, the "Cradle Skeptic," as I dubbed
him.  Similarly, grief was a matter that followed him from the early years
from the death of siblings, his father, etc. This was a man who'd witnessed
his own father's autopsy, so his view would be somewhat different from most.
I do recall the poignant letter he wrote about the death of his brother on
the "Pennsylvania" in which the young Sam Clemens merges grief with guilt,
and I suspect this notion might be worthy of further exploration.  I would
think an important book to look at is Hamlin Hill's MARK TWAIN: GOD'S FOOL
which discusses many of these matters, although word has it Hamlin now
thinks he may have overdone some of the aspects therin.

I guess I'm not sure which angle you're interested in--Twain's grief on a
biographical level or as part of his literary world.  In the case of the
former, his own words tell the story.  I've always been moved by his final
letter about the death of jean and his comments on the passing of Suzy.  In
the case of jean, he makes it clear he intends to write no more--he has
allegedly reached the end of what words can do for him. In the case of Suzy,
he finds some measure of consolation in the words she wrote about him.  i
forget where, but in one of the UCal collections of later works, perhaps THE
DEVIL'S RACE TRACK, there's some discussion on the lost ships in hellish
infernos being metaphors for Suzy's final fevers.   In the letter he wrote
about henry's death, I recall him feeling frief was a humbling experience.
I suspect one issue remains--how much can we infer in his fiction about his
attitudes towards bereavement?  I would think huck's many references to
death should provide clues or at least a commentary on the heavy presence of
death in 19th century consciousness.  What consolation is there when Twain
clearly never finds a benevolent God (despite that awful     TOUCHED BY AN
ANGEL episode which finds Our Author somewhat reconciled with God.)

I don't know, I haven't pondered this issue like you have.  So just some
notes . . .

Wes Britton