I don't know why it won't let you demote CC parts but we usually only use custom saved CC parts and don't use CC as such.
From what I've seen of CC parts they are mostly uneditable.
There may be a lock on CC parts which limits what you can do.
It may have something to do with tube and pipe, design accelerator, frame generator, bolted connection, this type of thing, they all tend to lock things down into their own assemblies.
Also patterns don't demote easily, normally have to destroy and rebuild.
Take a copy of the top level to try things out.
Select as much as you can demote to see what you would need to add to the sub in the way of CC parts, hopefully it's just a couple of joints.
Hope this helps some.
Not sure how well this would work for you but how about setting all the other parts etc to Reference within the BOM, then when you create your drawings and parts list they should not be included. You may need to manipulate how the referenced parts are displayed within the drawing to remove them but this should differentiate them from the main pipes.
I found a pretty simple solution to my problem. I just save another copy of my main assembly, grounded all the parts of a certain pipe run and deleted everything else. Then I brought that in to the main assembly.
Another question I have is if I can combine pipe parts in the BOM, for example in one pipe run I use all 2in pipes. Inventor lists each section of pipe as it own part. Is there a way I can combine those together and get an overall length on the BOM.
Not pipes but this blog post (great blog as well it should be said) may give you some ideas.
I found of way of grouping them in the parts list by substituting the part number column with the stock number, but everytime I found a solution another problem comes up. When I group them together it doesn't give the group an item number.
For example let say I had 3 pipes with item numbers 1, 2, and 3. When I group them it makes a new row and in the item number column it shows like this (1,2,3,) is there a way to give this group and item number