How can I diagnose why my ampr-ripd is not receiving the broadcast? Any tools?
Likely your problem is caused by a kernel bug that was introduced some time ago, I don't know if it has been fixed in the meantime or if it was defined as 'desirable behaviour' and left that way.
Some time ago I faced the same problem, and I could only fix it by adding the -r flag ("Use raw socket instead of multicast") to ampr-ripd. In multicast mode, which used to work fine, it simply did not work anymore on the Pi.
Rob