Bitcoins can't get lost. A wallet is a piece of software which manages your private-/public- keypairs to make it easy for you to interact with the bitcoin network (Send/Recieve coins).
Until no transaction is being made (either by yourself or through malware) your coins will 'sit at this address'.
Transaction can be traced between addresses almost 'infinitely'. But without knowing who is in charge of a specific address you won't be tell
whether the coins have changed the owner or are still belonging to a specific person.
No. All transactions on the bitcoin network are final.
If you own the private key of an corresponding address you can move the coins, if you don't.. you can't.