Well, there is some built-in OpenSSL function that takes a signature and DER-encodes it, but for the function that just signs there is no encoding whatsoever so my second "if" is needed and is actually hit like 50% of the time.

Haven't gotten any other rejected TX so I suppose everything is fine, the only question still open is whether that 0-byte "if" is needed for S as well (third "if" in the pseudocode) or just for R (I have them both right now but for S didn't get any hits inside the if so far).