Just to confirm, am I correct that my character's fear rating is less effective because they have Radiant Presence?

Yep.

Now I get it. It confused me before until I noticed that apostates have to decide between that or Disturbing Voice. They're equal and opposites, right? One makes you scary and less friendly, the other makes you friendly and less scary. Interesting that an Apostate has to choose one right at the beginning.

Radiant Presence should only apply to allies, Fear should only apply to enemies. Disturbing Voice applies to everything because it's very efficient for min-maxing.

Radian presence is explicit that it applies to everyone.

It confused me before, but it does actually make perfect sense that Radiant Presence stops enemies being scared of you. Think about a snake hypnotising its prey. Fear is a big part of your survival instinct, a group of apostates could convince a mook to lick a plaguebringer, but if he runs in fear on sight of it then they have failed to infect his army.

Which brings me to another question. Does it stack? Could a plaguebringer with a huge group of apostates wander through a town without people running in fear at all?

I'd say use common sense. The whole Fear thing only applies when you have characters actually trying to engage or interact with you. At least that's my take on it.

I doubt it will stack since it's a personal thing.