Can we propose mitigations whose sole or primary benefit is preventing disruption, rather than protecting an asset? What about enhanced drainage in a yard for nuisance rain storms?

Yes, a project that prevents disruptions in the case of a disaster, even if an asset is not damaged, could be eligible. For example, a hurricane might submerge the entrance to a bus depot, thereby affecting service but not damaging the facility.
Regarding nuisance events, please be aware that funding in this program is designed to minimize impacts or prevent damage from infrequent extreme weather events and other disasters, which generally means those with a recurrence interval of 10 years or more. A project that protects assets against exposure to typical weather, but has no benefit in a disaster scenario, is not eligible for resilience funding. However, if a project protects against a disaster, but also provides benefits in typical conditions, the application may present these and other ongoing benefits as reduced operations and maintenance costs over the project’s lifespan.