I had this problem, the car was stuck in the family inventory and I couldn't get it. I had one of the family members travel to France and buy a wine machine. When he reurned home all items in the inventory were then available. :-) Possible fix until patch can fix it better.
I am having the same problem with a custom world. It installed fine and plays and saves normally, but whe I go back to play the saved game it is grayed out and I get the same message. I also have all expansions and stuff packs. Any help Please.
I went the other way. I let them both go to elementary school, then a couple of days in High School. As soon as the parents aged to elder I aged up both Mortimer and Bella and had them get married. They now have Cassandra and Alexander children.
Yes, when you start the game and are at the pick neighborhood or family you have saved, to the right is "start new game". That gives you a whole new pristine neighborhood. When you save it then, save it titled under the family name or whatever situation you're playing. :-)