I'm thinking the reason titans barf is b/c they must eat family members/whatever Grisha was, to become a shifter
Even though i don't like the idea here some thoughts to this:
What if the Serum that Grisha gave Eren had this very function instead of
having to eat a family member he got it straight into the vein...
Now to what i originally wanted to talk about
Mikasa's and Levi's superior 3DMG abilities,
Eren's Titantransformation and his tempered memories...
we mainly think of Geographia as someone who has a memory-altering
ability but if we don't think of that as some kind of magic
but technology that may be the key to understand
the afore mentioned things.
Not just memories but the whole human physiologie could be
changed and considering how Eren, Mikasa and Levi are connected
that does kinda make sense. Grisha was (regulary?) visitig the Ackermans
who knows what kind of child-vaccinations Mikasa got
and Levi was trained by a special-ops Commander who leads a secret
anti-human 3DMG corps, who knows what kind of dope they get?
Then comes the whole titan creation thing. If some people have the
power to mess with something as complex as the human brain it's
believable that they can with enough genius/the right manual create titans.
and if humans are prepared with vaccation like those in Connie's village they
might even be turned into Titan's by a simple (telepatic) command or the roar
(right frequency) of the Ape-titan or something like that (coordinate) -
maybe even brought to form >100km wide circles transform and harden
to create the walls.
And "To You, 2000 Years From Now" might not be a reference to the story
being told in past tense but could instead refer to whatever information
Grisha found in the Basement. If we consider the story taking place
in the 15 - 16 th century that also ties everything together with
Germanic themes of Ice Giants and might be a hint that ~500BC Titans
were roaming and whoever rode to the future new that they would
awaken again...
Talking about telepathy and titans i might as well mention this:
what if titans live/are traped not just in a dream like Eren's dreams
and flashbacks from his transformations suggest but if the share it
considering that titans don't age that would make them the ideal
virtual reality gear.