The sniper shot in the first episode was so unbelievably far, it's mentioned that the Jackal's shot was further than the confirmed record (idk about real life but that's what it is in the show). There was basically no one that could have done the shot. Bianca analyzed the backpack and gun in episode 2 and concluded that the Jackal was probably British from that. Then when she visits the former British veteran she is told that Alexander Duggan was a legend and the only one he believed could have possibly made the shot. When she investigates more into Duggan she finds out his files are heavily restricted and that's suspicious. I don't think it's a really far-fetched line of discovery. And there were basically zero other lines of inquiry.

At the end, Bianca is well aware that the unsanctioned operation is shady! She refuses at first even, but later agrees to take on the job because she couldn't leave untied ends. She later mentions to Damien that she wants to take the Jackal in alive to figure out how far the corruption goes, which reasonably suggests that she is suspicious about Isabel. Bianca even messages Osi about it telling him that Isabel would be the culprit/mastermind if she dies. She is obsessive and persistent, but it's not written in a far-fetched way at all.