After grappling with this question for a day or so, I managed the solve the mystery of how to create a legal TID/SID for XD starter eevee with a desired PID/spread. i realized this when I decided to reread @Tux 's pokemon xd/colosseum legal guide and try to find a pid with xdseed. After reading through the thought occured to me to use the pokemon colosseum starter legit checker python script found here https://github.com/TuxSH/PkmGCTools/wiki/A-guide-on-how-to-make-legal-Colosseum-or-XD-Pokémon on eevee, even though it wasn't suggested in the guide, because the method of generation is around the same. Sure enough, after entering my desired PID, and selecting Umbreon as the pokemon I want the PID to apply to (I'm not sure it matters which one I select, but I'll just mention that), I found a TID/SID that made my desired PID eevee legal on PKhex
The question I still have is how did @Collector Togami manage to obtain a shiny eevee starter? Right now it seems like pure luck, but I have to imagine there is some way of calculating a legal shiny PID/TID/SID combination for starter eevee given than he rnged it. On TuxSH's guide, umbreon and espeon are said to not able to be shiny, but somehow this eevee is. I wonder why.
Also @Kaphotics I found a bug in PKHeX where the legality checker reports the PID as mismatched just because there's an invalid move. I'm not sure if there's some good reason for that recognition (e.g. an event pokemon with an exclusive move that has to correspond to a special pid), or if it's something that needs to be updated.
133 - EEVEE - BB9379B66955.xk3