Whether giving you an incomplete copy is legal or not depends on the intent and whether they are willing to honor the original contract as signed. If they gave you an incomplete copy to defraud you by getting you to pay for something they were not going to provide, that is fraud and illegal. I suggest that you ask to see the original contract that you signed. If the Agreement that you signed and the parties agreed to included 6 months mentoring then their failure to give you that is a breach of contract. If you cannot work out a business deal then you are left with litigation which is a lousy option.
I think the question is not so much whether it is "legal" but rather which version of the contract is enforceable. The other party cannot unilaterally modify a written contract without your consent.
Frank W. Chen has been licensed to practice law in California since 1988. The information presented here is general in nature and is not intended, nor should be construed, as legal advice for a particular case. This posting does not create any attorney-client relationship with the author. For specific advice about your particular situation, consult with your own attorney.
I agree that the main question is which document constitutes the enforceable contract. The effort of one party to alter the terms on a contract unilaterally can be seen as evidence in a dispute over the primary issue of which document is binding, but it is not, generally speaking, "illegal" in the sense of violating a section of the criminal code (although certain facts that are not present in your question possibly could). You may want to talk to a lawyer to get a sense of your options. Good luck.
I have been licensed to practice in the State of Oregon since 1990. I am not offering legal advice regarding your question, only general information regarding the law. You are not my client nor am I your attorney unless we sign a retainer agreement.