"So is the point that if I spawn a morph from inside Poser it becomes part of the cr2, but if I use SVDL's script or PLH or MM, I can create an external morph that I can inject?"

Exactly. The easiest way is to use SVDL's script. You create your character, you dont need to spawn the morph, just launch the script and you end up with 2 nice little injectors (INJ+REM). The Spawn Character python script by sdvl is fully fonctional but sdvl is working on an improved version which gonna be  a gem. Give it a try, it's really worth it.
Warning, there is no injection channels  in Aiko, morphs  are embeded, so you have to create a morphless figure (with MM4) and create the injection channels to be able to use injection technology.
As far as Miki is concerned, I don't know.

 "What is the relationship between PLH and MM? Are they somewhat similar?"
MM4 is dealing with morph management and PLH is designed to create injectors from an obj (you create your character, you export the morphed bodyparts or the whole figure as a  .obj file and PLH create the 2 injectors [INJ+REM] in DAZ format or standard format.) 2 different things really.

"I'm not sure if I understand what you meant about using Morph Manager 4 to create a morphless figure. Is that to remove any built-in morphs and just use one injection file to load into a morphless figure?"
Exactly. V3/M3...you can remove all the morphs with the REM injectors. Posette has morphs embeded in the cr2. If you want to have a light cr2 to speed renders up, you must remove these morphs data from the cr2.