And this is exactly the kind of thing they should have done to reinforce the image of the Lannisters and their alliance on top of the world. Another thing they omitted was the tell-tale line that clued Tyrion in to the secret of Bran's assassination attempt. The accompanying actions were there, so why not the words? Why not the comments from Oberyn & Ellaria regarding Tyrion's book to help bond the two characters, and give Oberyn some depth as a man of letters, and give the first hint of Ellaria's kindness, with her answering Sansa straight while their mates are trying to outdo each other in witiness (and show how mismatched she is with Tyrion, no matter how kindly he is toward her).
In their defense, this was easier to pick up on when you have the PoV showing he's drunk. The description of Tyrion's actions in the book don't really present a drunkard. Someone who is visibly wasted is hard to credit with subtly slipping poison into a cup.
Plus, Tyrion's retort to Joffrey that humiliated him and spurred on his mistreatment.

Not to mention, the whole thing was so crude and obscene. In the books, a similar scene would have had half the high-born grimmacing with distaste. The overtly sexual component of the performance only detracted from that royal atmosphere you rightly pointed out was missing.

One of the very significant complaints I have with the adaptation is their approach to sexuality, being both excessive and clumsy. It's one thing to portray a more overtly and openly sexual culture than the modern Western world, but the way they execute it is so awkward that they convey the impression of high school freshman who are reading a dirty part in one of Shakespear's plays, or the Miller's Tale and reveling in their license to curse or act crude. The whole length of the series has been like that, from casting a porn star as Shae (not to mention one whose heavy accent completely destroys any possibility of coming near her book characterization, leaving the audience to question what an intelligent, sophisticated & sexually experienced man like Tyrion sees in this shrill harridan who is constantly whining and nagging him and making his life miserable; book Shae expertly nailed the innocent-yet-bold, vulnerable and eager-to-please act that made Tyrion feel strong and bold and powerful. Being with TV Shae, how can he feel anything but henpecked? ), to the sexposition scenes, to adding a prostitute character that served no real purpose (I know fans of the show who remain unaware that all of Roz's appearances were the same woman). On the one hand, actors seem to revel in using normally unused obscenities and anatomical references, rather than using them casually as real people do when they curse, and on the other, they are all too fond of ridiculous euphemisms. See, for example, Daenerys' scenes, where sex is referred to as being "happy", which only comes across as a kind of wink-wink, nudge-nudge thing, rather than trying to delicately talk around an uncomfortable subject. Also, Daenerys, after her experience being sold for an army, would not ask her freed slaves to "make happy" people she wanted to get more information from.

Oberyn's & much of Tyrion's sexuality were only referenced in narration and dialogue. Their introductions in the brothels were completely superfluous and extra scenes with unnecessary actors, which could have taken the place of genuine characterization. Their sexual proclivities were simply aspects of their characters, and never something that got in the way of business for either man. For Oberyn, his sexuality was another way in which he ignores boundaries, and conveys his lack of conformity to traditional limitations, which really does add to his portrayal as a wild card in the whole mess. For Tyrion, it was a kind of weakness, to humanize a character who otherwise would have been a saint, between his intellect and unLannisterly kindness, as well as a sad kind of search for love. For either man to stop in the whorehouse before greeting his hosts at a new place, is absolutely ludicrous, but hey! Tits get people watching, amiright! And having the entirety of Oberyn's portrayal up to this point being a patron of prostitutes, including imposing himself on one who initially tries to demur, his line about decent people who don't commit rape is completely undermined and hypocritical. We've seen on the show how the prostitutes are kept in line in the brothels of King's Landing (and it's "the city" or "at court" NOT the fucking "capital"! The word 'capital' does not appear in ANY of the five novels, because they did not call it that! They don't have a capital city as we would understand the term! Also, they don't use the term military [or "mill-it-tree" in Game of Thrones speak], and any one of a number of anachronistic terminology that makes me wonder if Brandon Sanderson is diverting into teleplays), and that makes their johns not much better than rapists themselves.

Again, reinforcing the wealth of the family, and being something big enough that an old lady who's a bit unsteady with her fingers would not be able to miss. Also, I was watching the Hollard family necklace, and the stones were all there after Olenna did her hair bit with Sansa. So why did they bother with the necklace and Olenna's greeting?
Hey, that's a really good point.

Thank you. I've only been annoyed about that since he blabbed the Tyrell plans to a random hook-up in Littlefinger's pay last year! Not to mention it completely invalidates the respect Jaime will later show Loras, in contrast to Cersei's contempt. Rather than simply another aspect of her prejudices, instead, the show tangles in their almost-marriage to once again, allow Cersei to skate with an excuse for her behavior, and keep her humanized.

I was also under the impression that they had Littlefinger learn about the Tyrell marriage plans that way because they weren't bothering with Dontos. The whole point of it being Littlefinger who intercepted the plot was an early hint that Dontos was his catspaw, since the Tyrells are too competent to let their plans out of the bag that easily, and he was the only one Sansa was confiding in. But now, what the hell are they doing with Dontos?

"We paid for a giant actress, we're going to film the giant actress..."
Because they wanted to put Jaime in there, to play on the twisted family dynamic, and never mind that Jaime didn't really give a damn about Joffrey as anything more than an inconvenient appendage to Cersei.
Never fear, Cannoli's here!

Indeed. At my most charitable, I consider this whole show a lesson on the shortcomings of the medium.