I'm curious about how the "no people were here before" yet there are vaults?
It is conceivable..IF it is an Oldwalker Created Plane (there were some weird rules with that, ONLY the Oldwalker that made it could find it without help) and they made it specifically to build the Vault on. However there are some other issues with it if The People who Built it are as old as are implied...the Plane would have collapsed without the Planeswalker visiting it to maintain it's balance and existence (See Mirrodin Pre-mending).
It's a weird plane..but you COULD explain it in the setting.
And there are graves on the plane for Gissa to raise up their contents?
She only Necromancied once in the entire story, Her and Geralf could have just not been in the plot at all, Geralf did exactly zero. The "Random Outlaw Crew" was the worst part of the Plot to be honest. Oko and Satoru basically carried the rest of the crew (Tinybones did some minor Larceny). Gissa, Geralf and Rakdos were completely valueless, and Vraska is only there to Tard Wrangle Oko.
Literally did not show up in the story at all.
WTF is the "Coin Empire"?
They are the folks who made the vault
In Ixalan 2, Quintorius's plot is that he is researching a forgotten empire he Dubbed the "Coin" Empire because the most common relic he found were ancient coins, The Flavor Text in Ixalan refered to their race as Fomori, they are the ones who imprisoned the Inner sun of Ixalan, but Vanished..at some point near or before the Original Phyrexia was created.
And apparently double checking the exact details, they were a race of Giants from the plane Ir.
Edit : I forgot, Today I listened to BloomBurrow and Duskmourne at Work.
Bloomburrow is just filler with Ral looking for Jace and Vraska and getting side tracked by the fact that someone stole one of Maha's Eggs and fucking with one of the Elemental Incarnations of Darkness has consequences.
Duskmourne is probably my favorite of the plots thus far..because its a very simple rescue mission. The lore for why the entire Plane is a house actually made sense in context (Valgavoth got fused to the house as a way to seal him so as he expands his power the house expands), and he has just enough power to bait someone into the house through an Omenpath and since he controls the house once you are in you are in.
There are two big issues though...THEY DON'T FUCKING KILL VALGAVOTH so he is still going to be an absurd problem going forewards becuase eventually enough people will accidently fall into Duskmourne that he will be able to fully expand the house through an omen path...and it just proves that Elesh Norn's plans to take over the multiverse were fucking doomed because Phyrexians had no way to stop Valgavoth and Duskmourne from eating enough Phyrexian Souls to overpower Phyrexia.