He didn't say "here, sign this termination agreement." He said "here's my signed copy" which is lying by omission (on the assumption that your option c) is what happened). I don't know if that's a legal issue or just ethical, but it is not at all equivalent of them saying "Here's the termination agreement, you may review it at home but must sign and return."
I don't understand how you could know this, unless you were also part of this incident.
In terms of deciding how the legal system should work, I see a lot of upside and no downside to the sentiment "If you have a change to a document buried in a big contract and you actually agree to it, mark and initial the darn thing rather than just argue that the other party must have noticed or they wouldn't have signed". That's a comment about enforceability and a way to get predictable outcomes in court: parties to contracts that they actually agree to seem to gain a lot more predictability from the legal system if that's the standard, versus just punting a "party 1 said, party 2 said" question to a jury.
Do you think the manner in which he modified the contract is a deciding factor, then? If he had
marked and initialed the modifications, would that make it fine? Does he still have to explicitly mention it?
I ask because I'm confident that some companies would still sign the modified contract, and I'm interested in where we should draw this line. At some point, it's on them.
Remember that it's also possible for a bad actor to draw a strike line through a couple of words on their copy of a contract AFTER signing and then claim that it was that way at signing: being consistent about always following a particular formal process to handle changes gives you a lot more credibility if there's a later dispute about the agreement.
In theory, isn't that type of thing why you have a witness? Except if they don't read the contract, it's unclear what they witnessed...
I don't see how eliminating the second as an option makes it easier (your word) to engage in the first.
I'm not totally sure what "first" and "second" are referring to here, but to address the question: the only effect of such a measure is that it becomes much less work to pass out lots of complex contracts to people. You never have to worry about the other party modifying the thing, so you don't have to review them when you get them back. So it makes the job easier.
In practice, if this sort of behavior was common, companies would eventually become aware of the problem and take the (fairly trivial) steps to prevent it from happening. I don't see why we really need a legal framework to enforce a thing that they should be taking care of on their own.