I would like to do only 3 things:
1: Import an existing seed
2: Show the addresses for that seed
3: Sign a transaction
A wallet is automatically created in ~/.bitcoin/wallet.dat when bitcoind starts.
Presumably, you can move your backed-up wallet there to "import" the seed?
Not sure how or if multiple addresses are mapped to one seed, but check out
$ ./bitcoin-cli getwalletinfo
To send money (sign a transaction?), $ ./bitcoin-cli -named sendtoaddress \
address="a-long-string" \
amount=0.001 \
conf_target=20 \
estimate_mode=ECONOMICAL
But only after you decrypt your private keys (including suggested 300 timeout).See Grokking Bitcoin (2019) by Manning.