Can I build my seeds in a schema other than my target schema or can I split my seeds across multiple schemas?

Yes! Use the schema configuration in your dbt_project.yml file.

dbt_project.yml

name: jaffle_shop
...

seeds:
  jaffle_shop:
    schema: mappings # all seeds in this project will use the schema "mappings" by default
    marketing:
      schema: marketing # seeds in the "seeds/marketing/" subdirectory will use the schema "marketing"