Denti
You are quite right to suggest that a SATNAV backup must be agreed world wide. The two most important authorities for doing this are FAA and Eurocontrol. As far as I can see neither of these bodies have seriously considered the matter,let alone discussed it jointly. There are several alternative solutions, each withits own advantages and disadvantages. there may seem to be no great hurry, because relying on SatNav for increased traffic is still some way off. But because there are multiple choices to be made it may take timeto come to a conclusion--better get working at it pretty soon.
You asked how easy is it to jam ILS. I have never heard of any attempt, but maybe someone else does.ILS has very narrow beams(aimed to minimise multipath effects) which also increses signal strength. There is ground monitoring, and it will still be possible to divert on receiving a warning. Much less worrying than SatNAV.
Keith