All that's been clearly established to Roberto in the Road to Bibracte thread is that the magi of Mons Electi think that Apollodorus was murdered on Mons Electi grounds by a Guernicus hit squad led by Valerian. And that there is some vast Guernicus conspiracy, which he doesn't really believe, as confirmed by his first letter to Wirth. Some of the magi witnessed the murder, but there's been conflicting testimony between the magi... Your response of going to Val Negra is an attempt at converting knowledge you have as a player into knowledge you'd like your character to have. You need to do some work to connect the dots for Roberto. I left this as an exercise for you to do. In short, you're not going to achieve results anywhere close to what would be satisfactory to Roberto and/or you if you try and go to Val Negra now, his reasons now are too...ephemeral.
There's a reason why I saddled Mons Electi with the Tribunal and all the attendant preparations. Although it wasn't my original plan to murder Apollodorus as early as I did, the same information would have started trickling out that things at Mons Electi are not what they appeared to be. That the sweet deal Apollodorus offered the magi who joined came at a very high price. So yes, they are consumed with preparing for the Tribunal, while also knowing that their is a rot in the Order. The choice was which task gets tackled first. Obviously, House Guernicus is well practiced in maintaining this deception[1].
While Mons Electi is a spring covenant in reality, in practice they are a summer covenant with rich and vast resources. Summer covenants have problems different from spring covenants. A lot of times these are external and sometimes these are internal. Viscaria going overboard on preparations, committing to more than she can do and then leaving the pieces for the covenant to pick up is a good example of such an internal problem. The external problems are the Tribunal itself and the security measures Mons Electi wishes to undertake and managing the logistics of having almost 300 magi and their attendants in attendance. Mons Electi is close to the Rhein, Greater Alps and Provencal Tribunals. And there will be a Hermes Portal established by the Tribunal from Mons Electi to Harco to facilitate transport of attendees from across the Tribunal and the Order as a whole.
So, based on The Road to Bibracte, when Roberto arrives at Val Negra and asks for an audience for the Prima, about all he'll knows is that Apollodorus was allegedly murdered by Valerian. I've been very careful with the introduction stories for Tranquillina and Roberto about what could be known and to some extent when it could be known. Tranquillina knows a bit more, but that's because she was created with heavy involvement with me, choosing a one time Bonisagus of the Trianomae line who is in on the secret, and was in on it before Mons Electi was even born. She also had events transpire that gave her additional information to be used in the council meeting of her arrival. Roberto is an outsider, but comes from a well respected lineage and Santiago has allied himself with Ariel. Ariel is now testing her alliances. At this stage, Roberto is a pawn. Indeed all of the magi of Mons Electi are pawns to some extent. There will be opportunities to change that and become more powerful pieces as the saga progresses. Roberto just made his first move. The others have been making moves for a while in this saga.
Much evidence is gathered, but it's in Greek. An obstacle designed to limit the flow of information so that it could not be distributed easily before the 1228 Grand Tribunal. And it's circumstantial, requires conclusions to be drawn. There are some key allies, making them isn't an issue so much as uncovering them and making sure that they are real allies. It's pretty clearly that House Tremere knows all about this rot, and I think the characters know that they know. But House Tremere isn't moving yet... why?
And Murion, as Prima Bonisagus controls the agenda for the Grand Tribunal, there is quite simply, no hope that this will be put on the agenda. And she knows about this vast Guernicus conspiracy.
[1]Players and maybe even some characters can draw some conclusions from Roberto's entry story and Enter the Kangaroo. Did Calefacto, a known associate of the Prima really attack Roberto? What does one call a court which is convened to legitimize a verdict of guilt already established by manufactured evidence?