Is there any country where the 868 MHz or 915 MHz ISM band cannot be used?
What I wish to know is whether 915 MHz is currently allowed (license-free) in Europe?


No. It's not an ISM band in Europe.


  efficient use of spectrum for short-range devices within the 874-876 MHz and 915-921 MHz frequency bands.


"Short-range device" usage is very specific, that's not the same as "ISM band"; on the contrary. The European legislation that you're referring to leaves it up to the member states to define what an SRD is – and in European member states, those always (?) have been walkie-talkies, not things like machine-to-machine communications.

Maybe that will change, but it remains to be seen. In any case, you cannot assume this band to be currently available; that European guideline is pending national legislation.