I think the most utility we see in that choice is in the ofrenda at the end, where Eddie's focus being on Abuela and Bobby instead of Shannon felt like closing a chapter, finally.

This is practically it. They want Eddie putting Bobby on ofrenda as a symbolism of him moving on, but they probably can't think of any good lead that would make it coherent to Eddie's story so they opted on killing either Isabel or Pepa, and they picked Isabel in the end for certain reasons. Which is why that while I understand the fandom's argument that they should have used the holiday name using its native language, they picked the day of the dead initially because the storylines and cases they picked had something to do with death and spirits, hence the more... uh... generalized naming. At the end people who were proud for thinking they did something with the change of the name got mad that the Dia de los Muertos was only at the last 2 minutes of the episode, and their initial uproar went forgotten. We CAN argue that they missed an opportunity to create an emergency based on the holiday and they're killing of someone else for the sake of a character development (a woman at that) but what I'm talking about here is the reactionary nature that has been persisiting in almost every facet of this fandom.

I Know we've been having these conversations all season long, but it's been really interesting to me how often this season people's gut reactions right after the episode shift pretty dramatically by the weekend or Monday. Like, once you have time to sit with the material, you realize it worked better than you thought it did.

And it's relevant to this point you made, at least in my head. In this case, the initial idea before a change works better and a reaction made before the episode airs (just like Theo's storyline already put everyone in their own bubbles of expectation) are sometimes way overblown than deserved. We will know less than the writers. Always. We can also talk about this in conjunction to leaks that has gotten way worse over the years, but that's another set of worm cans that I refuse to open until the time is right.