Node form with fields loaded from another node or other entity
Isn't module Inline Entity Form exactly what you are looking for? Here is a quote about it (from the module's project page):

Provides a widget for inline management (creation, modification, removal) of referenced entities.
The primary use case is the parent -&gt; children one (product display -&gt; products, order -&gt; line items, etc.), where the child entities are never managed outside the parent form.
Existing entities can also be referenced.