I'm afraid I can't remember a huge amount from the geology modules from uni and so I'll take your word on the density of the stuff.
However, dense though it, is a lot of the granite used in old Cornish barns is 'field granite' which is fairly degraded and weathered, thereby allowing some moisture penetration. I think that internal condensation in old granite buildings is also a huge problem and maybe why this granite is perceived as being more porous than it is. My stonemason says that there's a fair degree of variation locally, granite from St Just , 4 miles away, is particularly porous, whilst St Buryan granite where I am is not so bad, but you still get some bad 'uns.
My feeling is that 300mm of granite against 100mm block is going to give plenty of protection to a fully filled cavity. Is driving rain really going to get through all that? Building control down here seem to think so but I wonder if they are open to persuasion if the argument is right. Should be talking to them in next couple of days so will be interesting to hear what they say.
As to the use of lime mortar, i'm undecided. If the granite is just a facing then it should'nt matter too much if i use a weak cement mortar and finish it off with lime pointing. There's always the struggle between cost, effectiveness and the apparent certainty that I'll go to hell for using cement.