I think there are two ways to fix the main problem discussed here.

1. Change inotify.c and make it return/receive a unique descriptor per client.

2. Wrap inotify's descriptor inside file-notify-add-watch, similar to
   how it's currently done.  This is what I was refering to as
   boxing/unboxing earlier.

The 2nd approach is problematic in the context of file-name-handler, so
I attempted to solve it the other way: Instead of a single number, use a
cons of

     (INOTIFY-DESCRIPTOR . ID) 

, which is unique across all active watches.  I.e. this makes inotify
work like all the other back-ends/handler.  Here is a first draft of a
corresponding patch, let me know what you think.