And what if the planned emergency landing (say at KEF for example) has to be abandoned because the airport is closed due WX or an obstruction on the runway?
Airports don't close because of the weather. If the weather is bad, pilots make the (sensible) decision not to land (unless it's an emergency). But it's not the airport that can decide to close because of weather, especially not with a mayday on finals.
If a runway is obstructed, well, most airfields (including KEF) have multiple runways available and in an emergency, pilots can and will land on a parallel taxiway if necessary - although it's not guaranteed that either the aircraft or the taxiway will be reusable after that. But it's better than an off-airport landing or a ditching anyway.
Other than that, I'm pretty sure that ETOPS rules will specify somehow that the ETOPS alternate has to be sensible and usable as an alternate.