even with a small chance of success (<1%), the expected value of passing the White House's pandemic prevention plan is so huge - trillions of dollars and millions of lives saved

You can’t just easily find projects that spend big amounts of money with 1500% rates of return like that? Trillions of dollars of expected value seems absurdly optimistic and not like realistic expected rates of return that you find in the real world for spending money on big projects. What usually happens is that most of the resources are wasted with underwhelming results or in counterproductive ways.

I don't understand what you think Holden / OpenPhil's bias is.

It’s not the kind of bias you’re thinking of; not a cognitive or epistemic bias, that is. It’s dovish bias, as in a bias to favor expansionary policy. The non-biased alternative would be a nondiscretionary target that does not systematically favor either expansionary or contractionary policy.

(If we want to talk about epistemic bias, and if I allow myself to be more provocative, there could also be a different kind of bias, social desirability: “you kind of value all people equally and you care a lot about how the working class is doing and what their bargaining power is” sounds good and is the kind of language you expect to find in a political party platform. This was in an interview and in a response prompted by Ezra Klein, but just seeing language like that used could be a red flag.)

I also think it's generally reasonable to form expectations about who in an expert disagreement is correct using heuristics that don't directly engage with the content of the arguments.

Yes, but:

1. Not when making high-risk grants, where the value comes from your inside-view evaluations of the arguments for each grant (or category of grants, if you’re funding multiple people working on the same or similar things but you have evaluated these things for yourself in sufficient detail to be confident that the grants are overall worth doing).
2. Not as a substitute for directly engaging with the content of the arguments, but in addition to doing that and as a way to guide your engagement with the arguments (to help you see the context and know what arguments to look at). Unless you really don’t have the time to engage with the arguments, but there are a lot of hours in a year and this is kind of Open Philanthropy’s job.
3. Never while framing as a good thing the fact that you’re deferring to experts instead of engaging with the arguments yourself, never while implying that there would be something wrong about engaging with the arguments yourself instead of (or in addition to) deferring to experts.