Drilldown search with Search API and Facets
I use CER,  "Corresponding Entity References" to achieve similar results, but not with Search_API.. for that, I just tell the search-page to render the "teaser", and modify the teaser display to include the entity_reference list of associated entities as links, that way, if you search for a "Kustomer", it will show you a list of Kustomers and each project will also appear as a list of links underneath the title, or visaversa, you can also use Facets to filter by taxonomy etc.. handy!