A couple of general tips if you want to modify treemacs beyond its builtin configuration options.

• If you want to get a feel for the codebase look to the functions treemacs and treemacs--push-button, they're general entry points for a large part of treemacs' functionality.
• Treemacs expects paths to be formatted the way f.el formats path - without the final slash. The only exception is the value of default-directory in the treemacs buffer so the correct directory is selected when creating files through treemacs.
• Each line in treemacs is a button (as produced by the button.el library), each button has a set of useful properties.
  • path the absolute path if the button is a file
  • depth number indicating how deeply nested the button is, used for indentation
  • parent the button's parent button, if any
  • index the button's part of the tags index, if it's a tags section
  • state indicates the type of the button. Will assume values like: dir-node-open, dir-node-closed, file-node-open, file-node-closed, tag-node-open, tag-node-closed, tag-node
• Reading button properties can be dangerous, since it only works when the call to button-get or a similar function only works when called from inside the same buffer the button is positioned in. In this case use either treemacs--safe-button-get or treemacs--with-button-buffer.