What if they crossed out a few clauses and signed, and the employer never noticed. Are you saying that because they begin working for the company, that represents the company's tacit acceptance of the modifications to the contract?
There’ll be explicit case law but it’ll come down to “would a reasonable person think the changes to the contract had been accepted”
The returned copy was accepted by HR via email (not sure if they ever read it or not)
What will be interesting is if this ever gets to be tested as not ever sheet in the contract was signed.
* https://www.adamsdrafting.com/making-sneaky-changes-to-a-con...
I don't think it has a direct legal basis, but like most things to do with contracts it builds the picture of intent and acceptance and understanding of what they were getting into.