Hugh,
Try a simpler test…
Create a dynamic sql adapter - select count(*) from table1 where field2 in (${field2InVlaues})
Do not use ? and instead use ${} as stated above. Once you save the adapter service it will create an input field2InVlaues. Run the service and give ‘a’ or ‘a’,‘b’ as input to field2InVlaues. It will work.