To start we need to first confirm what you have:

1) Master private key as you claim, it starts with "xprv..."
2) A .wallet file.
3) Individual private keys usually beginning with 5, K or L.

If you have "xprv.." I would assume it is a BIP32 wallet. In that case you have two options.

1. Download Electrum and import the key.
2. Save/download this website https://iancoleman.io/bip39/ and then open it in your browser locally (you need to open the index.html file probably). You can input the master private key there (BIP32 Root Key) and derive your addresses.