1/ I think the "key" doesn't have enough entropy to encode the text generated. This should be good evidence towards it not being a false solution. If your key c

1/ I think the "key" doesn't have enough entropy to encode the text generated. This should be good evidence towards it not being a false solution. If your key can't realistically encode enough information to "pick" out an arbitrary false plaintext, and yet the plaintext you have seems genuine judging by content, then you probably have a real decryption. Also the way the cipher was done is plausible to the time period (that is, it's something you can do with pencil and paper), and the steps seem like something a human would do.