OK, one more option (embarrassingly the most obvious one...):

5. Process the metadata (as S-expressions) already on the documentation web-server and directly communicate with that server from the editor.

• pros:
  
• Scheme-side implementation required only for the one Scheme we anyway use to build the server.
  
• No local Scheme client (server or REPL) required.
  
• cons:
• No access to additional Scheme specific introspection and documentation features; we can only present what we can define as metadata.
  
• Performance will be worse than for the local client; whether this is noticeable for the highly interactive features (hovering, Emacs-eldoc) needs to be checked.
  
• unclear:
  
• I'm not sure whether all interesting features incl. those defined by LSP can be handled by HTTP and I might oversee a limitation that we have in the HTTP-case that we would not have in the local-socket case.