Ambrose's plan is to allow Nyah to walk around after she's infected, but Nyah decides to simply kill herself after 20 hours rather than get infected. If the disease is contagious before the 20 hours, how did all the other people who may have gotten Chimera get treated? (Presumably Biocyte could have done it, but it would seem a bit strange to have Biocyte make some extra money in that way after their part in the situation). If the disease is not contagious before the 20 hours, how did Ambrose (who seems pretty smart at that sort of thing) not think of, or otherwise plan for, the possible suicide happening?
The disease in not contagious before 20 hours, and Ambrose took a pint of Nyah's blood as a backup (which he can sell), but left her free so she can start the epidemic, as revenge for her betrayal ("... you're going to take a lot of Aussies with you and make me a lot of money. ... In just a few hours you can be assured of going down in history as the typhoid Mary of Oz.").
This is explained immediately after the lab scene (Swanbeck and Ethan talk, explaining the contagiousness and the blood draw, and then Ambrose and Nyah talk, explaining the Ambrose's plan for Nyah to start the epidemic), and the Chimera-infected blood is analyzed by McCloy at the lab building.
This is also implied by Nekhorvich's plan: he injects himself with the virus and has 20 hours to get to Atlanta: he's not contagious before then, and they'll take his blood on arrival.
There's no suggestion that Ambrose thinks Nyah will kill herself or is trying to make her do so (not a Sadistic Choice), so this is presumably Evil Cannot Comprehend Good.
Ambrose may not have believed that Nyah would kill herself to save others, considering her previously-expressed selfishness given her chosen career.
You're overstating the importance of Nyah to Ambrose' plan. It's not like he was targeting some bigwigs specifically who only Nyah would get access to. His plan is much cruder - unleash the virus and reap the profit. And for that, given extreme contagiousness of the virus, literally any person would do. If Nyah does it, good; if not, oh well - just kidnap some bum, inject him with Chimera and release, same result basically.
