taxon_id = 558398765 # "Current" Taxon
brand_taxonomy_id = 854451436 # Your "Brand" Taxonomy
sql_query = <<-SQL
SELECT DISTINCT "spree_products_taxons"."taxon_id" FROM "spree_products_taxons" WHERE "spree_products_taxons"."product_id" IN (
SELECT "spree_products"."id" FROM "spree_products"
INNER JOIN "spree_products_taxons" ON "spree_products"."id" = "spree_products_taxons"."product_id"
WHERE "spree_products_taxons"."taxon_id" = #{taxon_id}
)
SQL
taxon_ids = ActiveRecord::Base.connection.execute(sql_query).to_a.map {|tp| tp['taxon_id']}
Taxon.where(id: taxon_ids, taxonomy_id: brand_taxonomy_id).all # All "Brand" taxons for all products in your "Current" Taxon
Task2 についても同様で、最後の行を次のように変更します。
Taxon.where(id: taxon_ids, taxonomy_id: category_taxonomy_id).all # All "Category" taxons for all products in your "Brand" Taxon