It would be nice to know more about how this mod works and how to change the way items are displayed/which items can be put in which slots.

I think the Readme covers enough information.

Probably a good idea but except for which item can get into the slots, it should be pretty much self explaining.
On the modpage there is already a short description which items can get placed into the slots :)

There are two ways to fix this:
Change the position of the third person item in your model json
I have to manually check the item in code and change the position

So basically Backslot uses a method to check if the item instance is allowed, so every item instance of the list on the modpage can get put in the slot without extra work :)
Most people don't read the mod page so I have no plans to edit/add the compatible items text on it.
You could make a PR for the items if they are not instances of the existing ones under network/SwitchPacket method isItemAllowed :)
Slot 41 is the backslot and 42 is for the belt