As the DM I couldn't be happier.
They basically sold samples of 4 different strains of a gnarly zombie virus, the lab used to make them, and all the notes the crazed wizard used to make them. And 1 of the party members almost died because of one of the diseases on the way back to town because he was out of spell slots, the disease kept lowering his CON stat, and part of the sickness was preventing him from getting the benefits of a long rest.
If a WMD is introduced in the first act, it must go off in the third act.
The PCs are just speeding this process along.
Exactly!
They're definitely speeding the process up in an exciting way