This paragraph confuses me: I've been playing with ASM80.com which is decent, but I can't figure out how to add my .com file into the CP/M drive it emulates.
Could you clarify this for me?
Try Z80pack on a pc, with the config tweak before build to activate Mostek ICE debugger.
However that won't resolve by itself your understanding of cp/m and com files. When you use asm80 or similar to create an object file , you then link the object to create hex or com files. If hex then hexload or hexcom will create the com files. Some assemblers eg z80asm can create the com directly on the disk.