They would have struggled a lot less if they had thrown out less.

The writers took out a lot, but they seemed to disregard the difficulty of writing plot to replace what they cut.

Dondarrion being just an extra sword is a result of the writers cutting the end of his story and then having to come up with more for him to do.

Maybe if they had let him die bringing back Stoneheart, we wouldn't have had to suffer through the "seven dudes try to capture a zombie for some reason" plot line. Beric would be dead, Thoros and Gendry would be with the Brotherhood under Stoneheart.

>For starters, it would've been another storyline/character that D&D would have had to wrap up before the end and they clearly were struggling with what they already had.

Absolutely not, as the beginning of Lady Stoneheart was the end of Beric Dondarrion.

I'm thinking much longer ago than that, if homo habilis tools have been found mixed among fish fossils: https://www.wired.com/2010/06/first-fish-diet/

I don't know how one would go about determining what adaptations allowed early hominids to catch fish.

Are there any traits that we can say are linked to humans' propensity to swim?

I know that humans (and most apes) have no instinct to swim, but humans do learn to swim and enjoy doing so. We've also been fishing for a long time.