i make IMBC with the pasturized egg whites all the time,
1. did the meringue whip up enough?
2. are the measurements correct?
3. it may just need more mixing
its hard to say what can be done, without knowing exactly what went awry.
as for what temp it can set out at, the melt point of butter is between 90-95 degrees F, so anythign hotter than that woudl be a no no.. you can sub out some of the butter for hi-ratio (never tried it with reg shortening) the melt point for hi ration is 98-110 or somethign like that, so it will be more stable but not exceptionally so