Making a colums total (SUM) with Views Popular Drupal 7 modules such as Views Calc and Views aggregate plus are not available for Drupal 8, so I'm struggling with achieving the following : 

Making the sum of a column (of one integer field of list of child nodes) per parent node, without any custom code. No matter what I do with the "Use Aggregation" feature , I keep on getting a list of the values of entities meeting my criteria and not the sum.
Your current configuration is doing the SUM of all numbers in that field but per views row, so you'd see numbers added up within each node if you had a multivalue field. 

To make it sum up across all results/nodes you need to remove the Sort criteria, that can throw aggregation off. 



I don't think your second question is making sense, you want to make a reference to a value calculated within views?  

Entityreference creates a list of entities, and the summed up value is not an entity, you can't reference it like that. You can create a Views block and show the summed up value on some content pages and wherever you need it. Is that what you're after?