Appache solr results multi sites search sorting based on specific feild
A quick Solution may be to use Apache Solr Sort Module.


  Per environment setting of grouping per content type/taxonomy/field
  value/... Allow for a custom amount of items per group (global)


But the best way is to setup a Apache Solr View :

Search API, has a Solr integration module, which will allow you to setup a View with exposed filters and full text search. With the flexibility of Views, you should be able to setup the view to allow the users to sort as they wish.