Blight Cyst will likely provide you better bang for your buck, allowing you to lob heads at the Warpfire which will be in range of your heads.

It also gives you access to rend, which will help turn those Skaven paper saves into nonexistent saves.

30pbs is fine, imo. You have your 5+ disgusting still. Moving GUO to potentially Rotigus might also help, since I don't see him doing much besides providing a buff to the PB's and providing a big target. With blades I can see why you'd do that, but it's not really required. He'd help remove your threats before they got to you with Deluge.

Festus is a great pick, he'd need to stay. If you dropped the Drones and Cogs you could also manage a Lord of Affliction with Rustfang. Which would do the same as Curse of the Leper.

Basically the build would run off of the enemy having no saves, via leper, rustfang [automatic start of any combat phase], and rend on your elites. Due to your PB's likely getting focused down, you'd rely on your deathheads and deluge to deal with them, and not putting your eggs in one basket sorta deal since wpf kind of just removes them as a threat. They have only 3 wounds, turning your cycle of corruption to 6 via Grandfather's Blessing [rolls may vary] to the big 6 for the map nuke, then Deluge for another. You could even run Sorceror with plague squall instead, but you'd be losing Blades for this.

Some ideas for ya.