With respect to relicensing, I assume that depends on a country's specific copyright laws. Ordinarily this license is as close to the public domain as you can be while still still having a license. So the protections against re-licensing are similar to protections for works placed in the public domain. For example you can't relicense Bach's works and then sue people for playing those works. So the burden is on the relicensor to show that their license supercedes the WTFPL. Unfortunately, the specifics of how this burden is handled in a court as well as treatment of the public domain in general are per-country (and sometimes subject to international agreements). Likewise with warranty, different countries will have different treatments.

What I don't quite get is why at this point you wouldn't simply place a work into the public domain or use creative commons or whatever. Maybe that is because different countries have different interpretations of "public domain."