Interesting point about IAN and RNP. Yes, the explanation to us was that the approach mode is (currently) only able and certified for straight in approaches, so it can't do curved GLS approaches or RF legs on RNP/RNAV approaches. There are not all that many RNP (AR) approaches in europe yet, the ones we used (LOWI and LOWS mainly) had RF legs and therefore the one size fits all approach in not approving IAN for RNP (AR) was probable the easiest way.
Quite honestly, never had a dual GPS failure at all, and even the single GPS failures were only for a few seconds at most. So i have absolutely no experience when the terrain display will disappear. Of course it would be a judgement call if one continues or not, especially if there is a longwinded low RNP missed approach at the end of the approach it probably would be better to discontinue earlier than later to get out of the terrain.