What if the session is automatically destroyed after 48h and something happens, say the funds get stolen/private key doesn't work? Will support be able to help the user then?
When session is destroyed - all data is erased. If you need longer session, you can extend it by clicking "extend" at the bottom of mixer page.
private key doesn't work
If you received private key to an empty address, please contact support. There is no way for private key stop working after time.
This is the reason why you should not use Chipmixer whilst on public wifi
Public wifi is fine as long as you use .onion address.