This is something you are setting in the parameters column, right? If so, this means that the value saved to the farmer form field is the value of the farmer_name Entity property. In that case, your expression would be instance('buzaaya-farmer-members')/root/item[farmer_name=${farmer}]/submissionUUID.
My recommendation would be to not set an explicit value. That means the farmer field will hold the system id of the selected Entity and all your lookup expressions will use name=${farmer}. You can also look up the farmer_name value by doing something like instance('buzaaya-farmer-members')/root/item[name=${farmer}]/farmer_name
Is there any progress on creating an entity list from a CSV file?
"Regarding uploading CSV file to EntityList, that feature is in the pipeline. "
It will be useful to have this. I have a list of entities already listed with a geographical location and would like to use these as entities for a longitudinal study.