@cgr
The greatness of Kui Ryoko's worldbuilding is such that she gives us a steady drip of hints and asides leading up to all the plot reveals which gives this manga's fictional setting that sheen of reality. It also means Dungeon Meshi is very fun to re-read...
-Post-digestive resurrection is referenced in a jokey panel from the very first chapter (Pg13) where Laius asks Chil and Marcille whether they've ever heard of anyone being resurrected after being turned to poo.
-In Ch19Pg10 the gnome scholar Mr. Tance talks about how in the dungeon "no matter what damage might befall the body, the soul cannot escape". This is backed up by Vol2 Monster Tidbits which tells us that souls which have lost their body become spirits which displace the souls of living people but the captured body eventually decays. Moreover Farlyn say in Ch11Pg8 that even spirits scattered by magic will reform after a while.
-In CH27Pg11 Marcille clarifies that monster souls are not bound to the dungeon only those of humans (a more accurate translation would be people, i.e. humans, dwarves, elves, gnomes, halflings and maybe orcs?).
-Regarding the level of damage a body can take before resurrection is impossible in Ch22 Namari explains to the twins that the chances drop after more than 1/13th of the body is lost but more skilled mages can still resurrect even badly burnt bodies. She also says trying to cure a possessed body might result in the mixing of souls.
-The egg analogies start in Ch41Pg25 and are mentioned again in Ch45Pg21, before being fleshed out further in this chapter.
So the assumption is that the dragons's soul was somehow linked to Farlyn's soul while she was ?partly/completely? digested and resurrected. Thus Farlyn's soul binds the soulmix to the dungeon and dragon's soul gives control of soulmix to the Mad Sorcerer.
Whether digestion is the only way to erase/release souls in the dungeon and the true nature of the soulmix(Bacon & eggs/emulsion/omelette/double yolked egg/?????) remains to be seen but whatever Kui Ryoko chooses it's sure to be well thought out, free of plotholes and delicious!