I sure as hell hope that pilots making weather diversions without HF/VHF contact follow the +/- 300 ft once established at 10nm off track. These rules are there for a reason and my safety is contingent on everyone else playing the game. If it is any consolation in all my years flying ETOP's I have never had to make a weather diversion that I couldn't get HF/VHF clearance for.
As for your 1st question, your ETOP's planning is just that. It is a planning tool to make sure you are compliant with ETOP's fuel planning. Once airborne (or should I say dispatched) then you are not bound to use your ETOP's alternates, you can choose ANY suitable airport. Infact our manual says that once airborne you can select ANY airport for an ETOPS alternate that has weather forceast to be above the expected landing minima. So in your example with weather above the ILS minima you could go there.