Body parts can be kept for analysis, yes! Bear in mind that if any of the central processors are kept undamaged then that is technically a living Null and it can signal its buddies for help regardless of whether or not it has a body attached to it.

Null arms aren't exactly weaponized in most of their shapes, and so there's nothing really on it to 'stab' per se. Maybe you could find one that's in the shape of something with bladelike arms? Still, if the laser cutters could be repurposed then those could also pierce the core. This will kill the Null!

Delight has pretty much cleaned out, unfortunately. There's not much to be found in her temple.