Decline the PhD offer during the deferral
I think this is fine in almost every case. If it weren't they would have you sign some legal document, but it is hard to see that happening. They might not be terrifically happy with you, but that would pass.
In a situation in which considerable university funds had to be spent (not just reserved) for you there might be complications, but then, any conditions would be clearly spelled out in advance.
I wouldn't worry about it unless something is said or implied in anything you sign.
Ethically, though, you should let them know as early as possible if you won't be taking it up later. But you want to keep your options open, also.

Generally speaking, though, in a situation like this, worry about your own best interests and assume that the other party is competent to worry about theirs. And since this is a friendly, rather than adversarial, situation, it will work out ok.