The value of nonlinear storytelling is not plot advancement but character development. It can help the audience understand why characters are choosing to behave they way they are.
This does not mean that the “writing is weak” as plot is only a small part of writing. Character motivations and relationships are key components which raise the stakes for the audience to actually care about the events and characters.
I’d argue in a story where the end is known (there are no surprises in Manhunt) this is even more important.
The water moving right through is probably a good sign someone is well hydrated! There can be total body water versus intravascular volume depletion scenarios, but not super likely unless someone is sick in other ways.
The xylitol itself may be contributing to GI water losses (it’s only about 50% absorbed through the gut, and osmotically draws water into/keeps water from being absorbed from the intestinal lumen). Depending on the SSRI, this could be exacerbated by bowel irritability that can present as diarrhea (sertraline is notorious for this).
Some folks also report more urination with intake of sugar alcohols like xylitol. I don’t have a mechanism of action for that, so take those reports with a grain of salt. (And also some glucose because sodium-glucose symport allows for water absorption without the need for an ATP pump.)