‘error trying to insert a Null into a non-nullable field’ means that one or more of the columns in your destination table aren’t being written to but cannot contain null values. You can modify the table to accept null values in all the fields so you have no problem starting the group (if you choose to add the 60th row).
Each block group must have a static number of OPC items, you can’t change them on the fly. But you can make sure all the extra rows in the database contain some known bad value that the PLC will discard. When you say ‘the query may return 5, 20, 35, or even 60 records’, what query are you talking about?
As for the picture above, it looks like your tags aren’t subscribed properly. Can you bring just one into a group and see if it shows you a good value?