So then we are back to my original qustion, "could it be a ploy". The assumption you are making is that the only thing he will do/has done is retrieve the heroin.
Please listen to me. I am not saying that is the only thing he has done. I am saying that is the only thing we have seen him do. If he did anything more, we do not know about it yet, because we only know what we were shown in the episode. And the only conclusions we can make at present are based on what we have actually seen up to this point.
There is an enormous and fundamental difference between saying "We have no evidence that it happened" and saying "It did not happen." And if you can't recognize that difference, then we are at an irresolvable impasse.
But we have no evidence yet to suggest he plans any such performance. Again -- I am not saying "I know he will not do this."An action that does nothing by itself unless we theorize on why he retrieved it. It may be the start of a downward spiral as he starts using again. OR, it may be used as a prop in order to convince Watson that he is using again, prompting her to decide not to move out. A result he has shown to very much prefer and would fit in with his exhibited use of manipulation. It would fit the character.
And yet, here you are posting paragraph after paragraph arguing against what was merely a simple suggestion of an alternative use for the heroin. We have no evidence either way on what he will do with it. My comment was one possibility that for some reason you seem to be pedantically arguing against.
Why didn't you just say, "we have no evidence for that, but it is possible" and leave it at that? Instead, you attacked the theory immediately with "She didn't see him take the heroin". Which implied that taking the heroin was the start and end of it, which is blatantly silly. Why have the scene in the first place if it won't be followed up on?
You do know how conjecture works?