I am using a apricot jam glaze to decorate the top of a Christmas fruitcake (made using rum soaked dryfruits and fed with rum after baking). This kind of fruitcake really lasts long due to alcohol in it. Now I am using a apricot jam glaze to put some nuts on top to decorate it. (could not get the nuts to stay in place during baking) Since glaze is made by heating jam with some water, does this glaze affect how long the cake will last?
No, the glaze will make very little difference. Jam is 60% sugar and sugar is an excellent preservative. To put it another way: if you find jam keeps well in the pot, the same should hold once it's on the cake.