1. ChipMixer does not seem to provide any mixer code to track the BTC sent to an address. So, is it possible to get back BTC from the same address, in a subsequent mixing?
You can't, because outputs are faster than inputs from a blockchain perspective; you can't receive chips which were created using your just deposited funds, because you can only receive chips which were made before your deposit.
2. How long the Vouchers remain valid?
Vouchers don't expire.