[SalesForce] Opportunity Product Quick Action Aura component with recordEditForm and inputField on lookup not displaying input fields when creating a record

Problem overview:

• I want to create a new record using a Quick Action (on Opportunity Layout for a new Opportunity Product) using lightning:recordEditForm.
• The component recordEditForm has lightning:inputFields for lookup fields Opportunity and Product.
• It also has lightning:inputFields for Quantity and UnitPrice.
• My client's org is Professional Edition (which means no Apex code!)

UPDATE: I found one error in my component. The OpportunityId will not be set correctly by using {!v.recordId} – that's going to be the Id of the OpportunityLineItem record (and it doesn't exist yet …). It is getting set correctly from the Opportunity.

The thing is: When I click on the Opportunity Quick Action, NONE of the input fields show up in the panel. The recordEditForm is completely empty – I verified this by dumping the form values using an event handler for onsubmit.

Component markup:

<aura:component implements="flexipage:availableForAllPageTypes,force:lightningQuickActionWithoutHeader,lightning:actionOverride,force:hasRecordId" 
access="global">

    <!--  Lightning action on the opportunity object. Clicking the Component's action button on the Oppty layout opens a panel to create a new Oppty Line Item.  -->

    <aura:handler name="init" value="{!this}" action="{!c.handleLoad}" /> 

    <lightning:recordEditForm aura:id="recordEditForm"
                           objectApiName="OpportunityLineItem" 
                           onsubmit="{!c.handleLoad}" >
        <lightning:messages />

        <lightning:inputfield aura:id="opportunityProductField" fieldname="OpportunityId" value="{!v.recordId}" 
                              disabled="false" required="true"/>
        <lightning:inputfield aura:id="opportunityProductField" fieldname="Product2Id" disabled="false" required="true"  />
        <lightning:inputfield aura:id="opportunityProductField" fieldname="Quantity" />

        <lightning:inputfield aura:id="opportunityProductField" fieldname="UnitPrice" />

        <lightning:button class="slds-m-top_small" type="submit" label="Submit" />

    </lightning:recordEditForm>

</aura:component>

Client controller:

({
    handleLoad: function (cmp, event, helper) {

        var obj = cmp.find("recordEditForm");
        console.log(JSON.stringify(obj));
    }
})  

The Quick Action LX Component when it renders:

Of course, the record is not going to save if the fields are empty, and here is the message when I click 'Submit':

"An error occurred while trying to update the record. Please try again.

Required fields are missing: [OpportunityId, Quantity]"

Any assistance would be appreciated. Some of the of SFSE posts mentioned that record creation doesn't work well in LX component lightning:RecordEditForm.

Another post indicated that the inputFields must be populated with default values.