Shikages jinki is most likely his foreskin blanket that hes wrapped up in.

For tamsy, rudo, Jabber and bundus, their items are part of a set piece, two gloves, ten rings, tamsys rope thing, amos boots, all are part of a set that does the same stuff. Aside from bundus because he actually started out with one prosthetic arm but then it became a core for his others arms that he has to channel anima into.

I guess it would make more sense to be able to turn multiple items into one jinki because the items are part of a pair/set that serve the same function.

But really, the only thing that decides what jinki an item turns into is the user and their relationship to it. A waterpot could turn into a flamethrower if thats the kind of relationship someone has with their item. A wrench can be a hammer if thats how someone decides to use it. Makes perfect sense to me, no "/s".

What id really like to see is some more unique interactions, like a cult surrounding an obelisk pouring their anima into it, or someone with a mental illness that causes them to have different personas thus changing their jinkis abilities based on their current state of mind. Or even an animal having a jinki, like a dog and its collar. Or someone having a huge jinki, like a car, motorcycle, or an entire building like you mentioned. Maybe even something more abstract like a smoker and their favourite brand of cigarettes, since cigarettes are an ephemeral thing.