So, this got me really confused, and would love some clarification.
Ghost syrup makes you Incorporeal (Ex), which says in the first line:
"An incorporeal creature has no physical body."
Do you become an undead or a humanoid incorporeal? Like what even are you technically when you are incorporeal, do you become like non-native outsiders whos souls are their bodies?
Secondly, since being incorporeal, you have no physical body, does the ruling of Incorporeal Possession count for leaving no body behind during possession?
"Ghosts, shadow demons, and similar creatures do not possess physical bodies. They are simply disembodied souls. They use the rules below to govern the interaction between their mental characteristics and their hosts’ physical forms. Such a creature merges with the host’s body and is unharmed when ejected. Creatures that use magic jar can often use the new possession spell instead."
You do not become undead. You are a humanoid (or whatever your type is) that has the incorporeal subtype. Do note that since you're still living, you need to eat and drink. Also, unless your gear is ghost touch or you have the Amulet of Grasping Souls, you can't hold anything. So you might want to figure that out.
Yes, since you're incorporeal, you follow the rules for incorporeal creatures.
You become incorporeal, per the Universal Monster Rules entry you cited. Looking at the description for the incorporeal subtype, you probably get that subtype as well (on the same grounds as determining whether someone has the extraplanar subtype). Your creature type does not change--you are still a human/dwarf/whatever you were before sipping that spectral sizzurp.