I think that Granger will have to really ponder whether he wants Jardines' banishment to be lifted. I think much of that will depend on what he finds when he returns to London. If Arthur is happy and content with Holmquist, Granger would be reluctant to drag Jardines back and potentially foul up the mix, especially since, as you noted, Jardines seems to be a total top, and so does Holmquist. But I can certainly see Granger arranging for Jardines to be posted elsewhere.
Jardines isn't an opium addict, at least not in modern terms. I think it would be more appropriate to say he was an opium user, and the drug helped dull the emotional pain inside of him. This is not unlike modern drug addicts, and like them, once the internal pain is removed or abated, so too should the drug use abate.
The other thing to keep in mind is that opium, as Jardines was doing it, is considerably less potent and addictive than it's derivative: heroin. I did a bit of research on this (as I do with most of this stuff...good thing I like it) and found the overall effects to be considerably more manageable when smoking opium as opposed to shooting up heroin.