If Sam wanted to buy shares, he can do it during the Exec...

If Sam wanted to buy shares, he can do it during the Exec Trading Window. But, if he had insider information then he would not be able to trade. I think it is clear that he does not have insider information because he was authorised to sell 4,363 shares on 1 August to fulfill tax obligations.

You mention an NDA as a possible block to Sam buying shares on market. I honestly don't see how an NDA would influence anything. What matters is insider information. If they have insider information, then that would block them from trading, but it seems clear to me that there was no issue with insider information on 1 August.