I saw the Ant & Dec DNA Journey (because of Reasons, none of which being personal choice!) and they did address this. From what I understand, they were part-way through discovering their ancestry when this all kicked off and suddenly Ant went off to rehab. Dec said (something along the lines of) that at that point he was seriously pissed off, but also that as well as being a business partner, Ant was also his mate. So it was weird for him to be furious at the selfishness of Ant's actions in terms of their careers and the potential fallout for Dec himself as well as Ant&Dec, but also worried about his mate and wanting to support him. It must be weird being the other half of an act like that when one party does something hideous. On the other hand, Dec also said that this was a complete bolt from the blue and that Ant hadn't confided in him about his addiction issues... yeah, right.I think Dec was almost certainly livid at Ant for doing what he did. Their careers are intertwined, and he was left in the lurch. He must have been aware of Ant's drink and drug issues, but perhaps thought/hoped he had some kind of control over it. He proved that he is capable of going it alone though, and maybe in the future he will become a solo presenter. We shall see. I agree they're not out of the woods yet, and I wouldn't be surprised if Ant falls off the wagon this year.
Personally I don't hope Ant falls back off the wagon, but that ITV get rid of the current little clique and get some new talent in. A&D could quietly disappear into obscurity with their millions.