@3monkies,
Plain granulated white sugar sweetens mascarpone. Just add a little at a time until it tastes right to you. Really. I know about this. I think of white chocolate as mostly sugar, so don't over do the sweetness.
Speaking of blueberries, you could use them in your cake or go for something totally unique and use kiwi.
The pineapple would have to be very well drained to avoid using a thickener. The tapioca does not really add a taste and works better than flour, so you might get a similar version of your father's recipe by using it.