Yes, the diodes are a (very) popular addition to modern mechanical keyboards. They prevent what's called "ghosting", i.e. when say 5 characters show up when you press 4 keys at once. Totally optional.
Also yes, you can reuse the PCB, provided it has one. It's possible it has a membrane in there, and not a normal PCB. Depends on your model.
Also yes, you can reuse the PCB, provided it has one. It's possible it has a membrane in there, and not a normal PCB. Depends on your model.