How to get referring items from a Entityreference field in the opposing item list? (back reference)
At the moment I only see 2 (edit: 3) options to solve the backreferencing issue without creating SQL request duplicates:


Using Views Field View for nesting a list of type A nodes referencing node type B via contextual filters in the second view to filter for each items nid (possible performance issue).
Syncing 2 Entityreference fields from both content types and maybe automate them with Rules and using the second Entityreference field for the back reference list in Views.
Using the CER module (I know you said no but), I think after testing that this module is the best option at the moment if you know that you want this connection synced "fix" and not individually changed.


First would have a huge performance impact if the lists and nested lists are long. And the caching modules required to compensate it are not ready yet for D8.

Second can have other issues sinces Rules not always work flawlessly and I am not 100% sure if Rules supports the needed triggers for this approach.

EDIT: Third is my recommendation. After testing around I was quite impressed about its simpleness and working warranty. It's not as flexible as using rules, but as I sad, rules is a story of ots own. So maybe you give it another try (CER).