The responseText element value maps to the position element value in the message sent,
How can I do an assign and update the initial message elements (responseCode, responseType, responseText) with the value received
from the web service response?
Can I use responseText (responseMessage) and position (messages) as some sort of key?
Yes, you can update initial values with the values received from external service.
For example, you can update each element with received values when responseText in responseMessage coincides with position in messages.