I'm glad someone brought it up. I've been waiting the chance to say something about that weird episode.
I was really pissed that my Inquisitor wasn't given the option to show the body to any of the ambassadors. I felt like my hands were tied.
When Teagan and Cyril came running to the war room, I just wanted to answer that hiding the Qunari soldier wasn't my idea. And to say I was sorry I allowed my allies to deceive them.
Glad that someone else also disliked what Bioware forced the Inquisitor to do. And we weren't even given the option to talk to them about the threat of Solas or the Qunari...t'was frustrating.
When the first Qunari body is found, the Inquisitor has no information other than the presence of the body and a bloody trail leading to the Eluvian. We the players know where the trail leads, but the Inquisitor does not. She does know, unless you are playing her as a fool, that the ambassadors are not her friends, and that they will spin the presence of a dead Qunari who came from an Eluvian to suit their needs.
Both Orlais and Ferelden, represented by Cyril and Teagan respectively, view the Inquisition as a threat. Quite rightly so. The Inquisition is an army with an expanding power base that is answerable to no temporal power. It has meddled in Orlesian and Tevinter politics, established military outposts, requisitioned lands to bolster its finances, and claimed for itself the right to judge and punish whomever it wants. Ostensibly set up to end the Mage Rebellion, the Inquisition has steadily expanded its mission to include "fighting evil" and "maintaining order" at the point of the sword and regardless of existing authorities. It has done so for "good reasons", but that doesn't make it any less of a threat.
The only difference between Cyril and Teagan is that one thinks it can be subsumed into the Orlesian state apparatus, and the other knows the Inquisitor won't quietly become a Fereldan or Orlesian subject and so argues that instead it should be disbanded.
So if Cyril sees the Qunari corpse, he'll talk about how the Inquisition is in over its head fighting the Qunari; and Teagan will talk about how the Inquisition is drawing Theadas into "another war". Neither will be happy about letting the Inquisitor do her own investigating, and both will use it as ammunition against the Inquisitor, because absence of information has never stopped someone in their position from spinning stories.
So the Inquisitor could bring in the ambassadors and show them the body, but it would probably just make things worse.
More generally, it's made clear over the course of Trespasser that the Inquisition cannot continue as an independent army that careens about the land searching for wrongs to right. It threatens too many powerful interests. Nothing the Inquisitor can present: the threat of a Qunari invasion, of Solas destroying the world, or anything else, will allow the Inquisition to survive.
Making public that Solas, who was a close ally of the Inquisition, is now trying to burn down Theadas, is probably the worst thing the Inquisitor can do. Even if the Inquisitor hated Solas, she will be tainted by association. Bonus points if she's an elf: "We're elves, together we're two elves, then we need to be elfy because we're elves for other elves and elf, elf elf."