"Ghost Boster" (the one where the singer gets food poisoning and it makes him angry) is about a man becoming a vegetarian.
The references to ghosts being in the food are meant to be references to how the food he eats used to be parts of animals with some degree of sentience, and he's getting weirded out about it over the course of the song. Much like how some people who decide to become vegetarian "cheat" to a degree by eating only certain types of meat, typically from non-mammals (poultry, fish, invertebrates, etc.), the singer starts by simply swearing off mammalian meat, only to find out that chicken also comes from animals. The line "Why the fuck is all the food full of ghost?!" is him questioning why so many dishes are meat-based. However, the song isn't completely on his side, as some of the things he decides to do (calling the Ghostbusters on restaurants, telling them to destroy the city (possibly out of anger at how prevalent meat consumption is)) would be more in line with the actions of an Animal Wrongs Group than just an average vegetarian. Ghost Johnson is responsible for the city's ghost infestation.
When he is first introduced in "Ghestbest," Ghost Johnson brings Grandma back wrong as a skinless zombie. Grandma probably wasn't the first person he tried to bring back from the dead, considering that he advertises his resurrection services. The singer is likely just one of many people in the city who've had to deal with departed loved ones being turned into undead or supernatural beings courtesy of Ghost Johnson's magic drum playing.The Ratman is somehow related to Jerma's rats.
Possibly a Giant Rat who managed to gain a humanoid form. Such is the power of rat."Ghestbest" happens at least one year after the events of "Ghostbusterz".
Both songs involve a dead grandma, so of course this would be the case.- Adding onto this, the protagonist of "Ghestbest" is the same guy as the protagonist of "Ghostbusterz." In Ghostbusterz, the singer is motivated by getting his money back from his dead grandma. The singer's grandma not giving back the money she borrowed, alongside hiring the Ghostbusters to revive her, ends up having a major negative effect on his funding, resulting in his dour state by Ghestbest. Presumably, the Ghostbusters revived Grandma properly because he paid them, while Ghost Johnson, despite claiming there are no refunds, wasn't paid and thus brought her back as a hostile zombie.
- Them being the same person was part of my original theory, but I didn't mention it as I thought it was too obvious.
The narrator in "Ghosterboster" is one of the ghosts in the storage unit
The "landmine" is actually a Ghost Trap.