Hi Erik
I have been trying to merge the fbx file. I couldnt find append unless that is just a term they use for merging, no buttons or menu items in the UI or manual. Opening the pose file wants me to clear out the scene and save it so that is not really handy.
Your idea of using Mia was super, I am having exactly the same problems with that character. My character is just max bones characterized and with a control rig so there is nothing unusual about the rig MB is doing all the FK and IK work 
Really weird when I try to load the poses via merge the pose directory icon flashes as if it wants to display the files but fails. Maybe it is something to do with my NVIDIA display. According to google I am still the only person in the world with this problem so I am sure I am missing something.
Thanks for your offer to check out my file that is very kind of you. I assure you though it is a very unspectacular rig!
The steps I go through
Select which parts of the rig I want to pose and add a pose at the default/rest position.
Rename the pose.
Select and move the bones/effectors into the pose and create and rename.
Test the pose, works every time.
Select the poses and choose Save Selection from the file menu.
Save as FBX with everything checked except lights and cameras, saving with or without take001 makes no difference.
Delete the poses in the pose controls window.
From the asset browser or from the file menu, dragging the file into the scene and merging like I have seen on tutorials fails, dragging into the pose control window or over the folder in the pose controls or the pose folder in the navigator all fail.
Saving the poses with the rig parts selected only merges those parts of the rig into the scene not the poses.
Thanks for all your help so far, maybe you can see the obvious thing I missed.
Cheerio