Grinding up cured silicone to use a filler does in fact work, but as you discovered, it needs to be similar cure-type. Additionally, you want the silicone to be as clean as possible.
Vacuum degassing after mixing everything up is also a really good thing to do if you have the resources. Small air-bubbles like to cling to the chunks, weakening the mold.
I have a 5-gallon bucket full of dead molds and flashing scrap. One of these days, I'm gonna buy an old-timey meat grinder and use it only for grinding up junk silicone.