Yes 😉
On the question of making a manifold solid...
When you add in the two 'transformations' [as I showed] there will almost certainly be internal faces as 'partitions'.
These will stop a group being seen as 'solid'.
The definition of a solid 'group' is that all of the edges within it must have two faces - no fewer, no more.
So an internal 'partition' face messes us up because some of its edges will have three or more faces.
Also 'flap' faces will have edges with only one face, and a faceless edge obviously doesn't have two faces at all...
It should also contain only faces and their edges to be 'safe'.
Once you've exploded the bits into one collection of geometry and grouped it all, if it doesn't show as a 'solid' in Entity Info, then you need to edit it.
There are scripts that show or attempt to fix such issues - however, in a simple case like this you need to try a few things first - use X-ray mode to see inside and perhaps remove unneeded partitions by erasing their edges that join onto external faces; also you can use the section-tool to cut the object temporarily so you can see inside it, then select+delete internal faces, edges etc: running an 'erase coplanar edges' tool on the results also simplify it. At each step check the Entity Info to see if it reports a 'solid'.
🤓
