The eggs will get cooked well enough. If you're making IMBC the sugar syrup is cooked to 248 degrees--which will make your egg mixture well over the safe cooking temp for egg whites (they would no longer be considered fresh eggs at that temp they are cooked). If you are concerned about it you can buy pasteurized shell eggs or pasteurized egg whites.

I have an IMBC recipe posted in the recipe section of the website using CK brand meringue powder (don't use the Wilton, it leaves a funky taste).

Sounds to me like your cake is probably too soft and your icing too hard!!!
I don't think it has much at all to do with the age of the cake as I ice them when I need to!! LOL sometimes that several hours and sometimes a day or so later... I do so many i don't have the luxury of waiting sometimes ( especially last minute orders!!) I'd consider softening the icing and try a new cake recipe. atleast until you get good at icing them!, if your using pure BC try heating it up a little. just my 2 cents>>> LOL
Mike