The Don of Detroit (I believe it was Detroit in the novel) is giving his speech about drugs and Tattaglia is seen whispering something to an underling. Any significance about that?

I think the directorial significance about the whispering was to show that the Detroit Don's speech was empty. The meeting wasn't about selling drugs to "the dark" people. It was about finding a peace between the New York families. Tattaglia and Corleone were the ones who had to decide which moves to make.

I always wondered about that too. I think it was just directorial license. Perhaps FFC wanted to create a little bit of tension for the audience by having Vito's arch enemy doing something unusual as if something was about to happen.

...our, it might have been another hint that Tatt was a parvenu, as when we saw him combing his hair at the beginning--hardly a thing a serious Don would do at a Commission meeting.

Correct me if I'm wrong but I thought Tattaglia wasn't really respected among the other Dons, and they basically thought he was a scumbag. It could be inferred to show that he has no respect for Don Zaluchi because he's not really listening to what he has to say (as presumably the other Dons were) but chats with his henchman (who, I presume, is his Consiglieri) while Don Zaluchi is talking.

The novel says that Tatt was looked down on by the other Dons because he was a pimp, and because he let himself be led around by Barzini.

The Don you're referring to is named Joe Zaluchi in the novel. Zaluchi was a close ally of Vito Corleone, and Tattaglia was Corleone's enemy, and therefore probably also Zaluchi's enemy.

They also blamed Tattaglia for starting the war by his inefficient murder attempt(s) on Vito. They felt that if he had been successful at the shooting, or later at the hospital, Sonny would have been forced to make a deal, and the war would have been averted.

There's no question in my mind that Barzini arranged for the don from Detroit to make his speech which set the conditions for the compromise Vito was going to make. Barzini also must have told Tatt what he was up to and based on those conditions he would be forced to make the peace with Vito like it or not.
Accordingly Tatt didn't need to hear the speech, so he was probably making arrangements for a post meeting hooker.

At the end of the Detroit Don's speech there is a little moment that I like where Vito gives Tattaglia a stare and you see Tattaglia squirm in his chair a little bit.

Considering that the book talks about how close Vito and Don Zaluchi were I think it's highly unlikely that the speech was orchestrated by Barzini. It certainly furthered Barzini's ends, though.

I don't see why that speech couldn't have been orchestrated by Barzini who virtually controlled the Commission by that point. He could've said to Zaluchi that "drugs are going to happen whether you like it or not, perhaps you can help me convince Don Corleone." The problem is that the film doesn't highlight any such prior relationship between Corleone and Zaluchi.

The speech by Zaluchi in the film is taken directly out of the novel. If it was orchestrated by Barzini, Puzo would've mentioned it. Puzo states that "Zaluchi disapproved of traffic in drugs" and that Vito "could count on Zaluchi for support."

Doesn't mean that you're theory isn't plausible though.

It could be one of the things in the novel that is left to the reader to interpret themselves. Was the passage about Zaluchi being against drugs and that the Don could count on him for support before he made the speech?

It's stated by Puzo when Zaluchi arrived at the meeting in the beginning of that chapter. Then a few pages further he makes his speech.

I don't remember if Brando showed anything in his acting that suggests that he was taken aback by Zaluchi's speech. I did note that Tattaglia almost had a smirk on his face following Zaluchi's face though.

Zaluchi was honest. Maybe he really thought his suggestion would help smoothen the situation between the NY Dons.
Vito knew that he had no chance but to agree on the drug proposal. The only question was under which conditions. And his main goal was to bring Michael home safely.

The implied threat in the don's peech was this : He too was opposed to drugs and he even paid his people extra not to push them the problem was no matter what he did at the end of the day, when it came to drug selling "they can't say no." In other words, the message was no Don had the power to stop his own people from being disloyal. It was purely business, and drug trafficking was going to happen no matter what. The implied theat to Vito was "get on board with this or you will be losing your street muscle." Once that was made clear, then the terms of the compromise were laid out.

As Danito states above, Vito's motivation for the Commission meeting was to achieve a peace of sorts so that Michael could return home. That was it. We don't know for sure if Vito kept his promise once Michael returned, but it was likely he did so in order that his and Michael's plan to wipe out Barzini and Tattaglia could occur as planned.

Obviously Vito kept his promise, and in fact this was still the policy in New York in GFIII when Michael was keeping Zasa from rising up in the comission because he was selling drugs in th old neighborhoods and to kids.