Out of curiosity: the subtraction of 1 from the intermediate 6P private key (I assume after conversion from WIF to hex format, otherwise the WIF's checksum would break) is an intentional obfuscation step?

Your recipe is already a bit complex. But why this subtraction step? Didn't you trust your initial passphrase1 and encryption passphrase2 enough?

Simple brainwallets (by human standards) are in many cases fundamentally flawed which has been proven. You simply can't use anything that's online available as source for a simple brainwallet (simple in terms of taking only the SHA-256 of the source phrase as private key).