I would recommend do a test of the procedure without parameters by inserting test values directly to ensure it works. This will do 2 things, first; confirm the procedure works, and second; indicate the issue with min/max is in the QueryRecord. While testing flow, you should be able to see what the values of min/max are by inspecting the flowfile/attributes.
Excellent progress. Can you try doing a single select to get both values?
Like this: SELECT MAX(ID),MIN(ID) FROM FLOWFILE
If you could do that, then you could extract those values into 2 separate attributes, then feed that flowfile to the next processor in the flow. Another alternative is to extractText for each value to an attribute, then MERGE 2 flowfiles into 1.