Sorry I didn't answer this when you first asked, I'll try to make up for it this time.
1. *Methylated Spirits is also Denatured Alcohol. *The reason I mention this is because lots of places, and I'm sure that Spain is one of them, might tell you they don't have it because they don't call it Methylated Spirits. *Denatured Alcohol is used just about everywhere, and so you just need to know the various terms for it. *You might look in your dictionary for "el alcohol desnaturalizado" or perhaps "el alcohol del metilo". *Methyl alcohol.
2. *I can't find a comparison on the Internet, so I am relying on memory (not necessarily so great, I'm afraid), but I believe that LPG/Butane burns hotter than alcohol, and it isn't necessarily significantly safer than propane (except that it doesn't explode, of course!).
3. *Your big problem is the storage of the gas bottle. Australian regulations require that the bottle be mounted on the outside of the boat away from any hatches, usually, I notice, on the aft rail. *If that is not acceptable to you, is there any place that you can put a prefabricated locker for the propane bottle? *Supplier of propane accessories
How about CampGas bottles, small butane/propane bottles you return empty and pick up new full ones in the EU? *Not quite so large and bulky, easier to make a safe place for them.
Not knowing what kind of boat you have, I can't come up with more ideas, but most boats have found some way to safely carry propane, even on small boats. *Some things to think about, anyway.