This issue has been resolved. Instead of setting up the .xsd as above I've set it up as seen below.
<xs:element name="orderlines">
<xs:complexType>
<xs:sequence>
<xs:element name="orderline" maxOccurs="unbounded">
</xs:element>
</xs:sequence>
</xs:complexType>
</xs:element>
This allows me to setup a variable in the logic app as shown below as input for the "Orderline"