Backsweetening with whole fruit instead of juice might be tough to gauge the proper amount. Personally, when I add fruit to a fermentation I will freeze and thaw it once or twice. With most fruits this will soften them up, then you can just kind of smush them by hand in your freezer bag and dump them in. Except for cranberries - those suckers can take a freezing like no one's business. For those I usually thwack them with a meat tenderizer to bust them open.
I've never sanitized fruit before adding it, but I've never added it to a stabilized beverage before, either. I don't know if Star-san is going to help much, to be honest. I think winemakers are more likely to add Campden to the crushed fruit, so if you feel the need to do so, then I'd be more inclined to go that direction.
Using a canned sterilized puree might be a safer place to start.