[SalesForce] How to call multiple functions from “change” attribute in Lightning Component

Without the full code sample it's tough to tell why Lookup__c field isn't present, but I'm betting it's because of object references.

This bit here

component.set("v.oldItem", component.get("v.item"));

Associates the v.oldItem object with a reference (in javascript not Lightning) to the v.item object.

Since you said you wanted v.oldItem not to change when v.item changed, I'm betting this is not what you wanted.

Try doing this as a cheap and dirty copy.

component.set("v.oldItem", JSON.parse(JSON.stringify(component.get("v.item"))));

See if that works.

We do not clone objects in Lightning, that would get messy perf wise, so primitives are done by value, objects (and arrays) are done by reference.

Here is a small experiment as a proof of concept to emit events from lightning component and handle in VF

<aura:component>
 <div> Hello World !!!! </div>
  <aura:registerEvent name="myevent" type="c:myEvent" />
  <ui:button label="fireEvent" press="{!c.fireevent}" />
</aura:component>

The JS controller file

({
 fireevent: function(component, event, helper) {
    var myEvent = $A.get("e.c:myEvent");
    myEvent.setParams({"data":"Test"});
    myEvent.fire();
  }
})

The Event file

<aura:event type="APPLICATION">
 <aura:attribute type="string" name="data" />
</aura:event>

The application file

<aura:application access="GLOBAL" extends="ltng:outApp">
  <aura:dependency resource="c:myComponent" />
</aura:application>

The visualforce where event is handled

<apex:page showHeader="false" sidebar="false">
<apex:includeLightning />
<div id="lightning"> Hello world VF ..!!! </div>
<script>

    $Lightning.use("c:myEventApp", function() {
        $Lightning.createComponent("c:myComponent", {}, "lightning", function(){
            $A.eventService.addHandler({ "event": "c:myEvent", "handler" : visualForceFunction});

        });
    }); 

</script>

<script>

var visualForceFunction = function(event){
        var myEventData = event.getParam("data");
        console.log(myEventData);
    };

</script>

DISCLAMIER : I have not tried inside service console but this should help you get started .For ISV apps since this is not documented anywhere ,consult SFDC security review team or salesforce lightning component product team to see if this is right way of doing this .