At first glance and not complete answer:
1. you cannot intersperse FileMaker functions inside the ExecuteSQL 'as is', you must use the concatenation:
" SELECT COUNT( " & GetValue($_field_list; $field_list_rep) & ") FROM ...."
2. if you are looking for "fields which do contain data", you don't use NOT NULL, you use NULL
So, I guess I'm confused what you are searching for! Think it through if you had to MANUALLY get the data you want.
Resolved and Fixed. Thanks for everyone's help.