MtMirror and Compose work like a charm for me on the neck, chest, abs and hip, but I'm having a problem with right/left body parts. How do I set up the base file in Compose? Do I merge right/left collar and save that? How do I set up the morph file? Do I pair up the right/left collar or just use the morphed collar? Also, if I do merge the two groups in the morph file, don't I have to break them apart in my modeling app (Blender) in order to load the morph target into the new character in Poser? I've read the available tutorials and they aren't clear on these points.

|Do I merge right/left collar and save that?|

Not merge, save as an OBJ, with just those two groups.

|How do I set up the morph file?|

Exactly the same.

|Do I pair up the right/left collar or just use the morphed collar?|

You need both groups, the unmorphed can be the base/ usually is. If you did not have the other side in your modeler, you can use Compose to assemble the two optical isomers from the morph and the original.

|if I do merge the two groups in the morph file,|

Rather than merge, just select the two groups and save it as an obj file. (In Compose, it is a pain that it makes you type ".obj" every damn time.) You just want an OBJ file with a symmetrical group and only the symmetrical group. For MTM's purposes, a group is one Poser group - for chest and other midline groups and the two sides for the distal ones.

|don't I have to break them apart in my modeling app (Blender) in order to load the morph target|

Yes, you have to get the separate isomers back. MTM used to only save as stripped "v" lines and I am not sure it still gives you something you can use in Compose to isolate the sides. But it is easier than you think.
You either already have the morph you did separate - if you test in Poser as you go, or you can save it as a separate group from Compose when you assemble you stock for MTM.
Just let MTM save you morph as "v" only. Open it in in EditPad, and
if your mirrored side if the left - cut the block of "-v"
if your mirrored side if the rght - cut the block of "v"

if you forgot to save the side you did the morph on
paste the cut into a new document.

You may have a problem with the buttocks.
Compose tends to invert l&r in the saved file.
MTM says it mirrors, but you get nothing.
Look at your base stock and your morph stock - make sure the order for the v/-v is the same in both.
It is easy to fix in EditPad if they are inverted in the morph stock.

You are the man. I used and loved your morphs for V2, but now it's time I did these things on my own. Thanks a million.