So this is what you need, basically:
-snip-
If you are only interested in the keys and the corresponding addresses are known, then you can also inspect the old wallet on an offline machine, right?
Yes, but it depends on what you mean by "
inspect", if through simple open as text, even if the wallet isn't encrypted, the keys wont be readable.
If through "
hex viewer" tools, it's possible with unencrypted legacy wallet files and master keys for descriptor wallets.
If encrypted, you'll have to rely on wallet.dat-compatible tools.
I wouldn't recommend it unless the wallet.dat is corrupted though since you can still use the instructions on an offline machine.
Bitcoin Core doesn't need internet to restore or load wallet.dat files to be able to export the master keys/private keys.
What does Bitcoin Core actually do when restoring old non-HD wallets? Does it convert them into HD wallets with the old key pool added?
It doesn't convert, "
Restore" simply creates a wallet-name directory inside wallets directory and copy the selected wallet.dat file there, then attempts to load it.
The only update that it'll do is to update the keypool to 1000 with the old keypool intact, it'll remain non-HD unless imported with an HDSeed.
There are other commands that will upgrade, import HDSeed or migrate (
to descriptor) the old wallet.dat file.