How to build upon Autocomplete widget to fill other form's fields [#3329029]

Problem/Motivation

I have a Fieldable Panel pane with an Entity Ref field and some others.
I want my user to use the Entity Ref field to reference a node and have information from that node update the other FPP form's fields upon selection.
I am looking for pointers as to where I should start acting.

I am thinking of using a form_alter for my FPP form to add an Ajax property to the Entity Ref field. However I do not know the order of execution of Ajax calls. Can I simply add an Ajax call and the value of the Entity Ref field will be present in the form state, so I can use it, going further? Or do I need to alter the Autocomplete Ajax logic in some ways (i.e. build upon the behaviour).

To be more specific, this needs to happen as part of the Panels IPE module, so we are talking about an FPP form that is the result of an Ajax call. Can some more Ajax be added in this context ?