You won't lose the money obviously, it has nothing to do with Chipmixer as unconfirmed transactions based on the mempool.
Furthermore, the mixing is done even before you deposit bitcoin to Chipmixer and you will get the private keys after the transaction gets confirmed, for transactions less than BTC10, it just needs 1 confirmation.
A little bit confuses, you have sent bitcoin to Chipmixer but after 28 hours still unconfirmed.
Yes, there is a button to extend the session. But what about 1 of the 4 addresses? is it refers to 4 private keys or you have sent bitcoin 4 times to Chipmixer? Anyway, how is it going, is it done?