This is 9.1 version where Append sub-query was not supported that time. Append sub-query is supported from 9.1.03 version.
I thought of this and sorry to make you try it.
Back to original discussion now -
At this point we are clear about the fields that used in master query i.e. Submit Date's month & year calculated fields which are concatenated using advance function, status except closed\cancelled\resolved, and the number of incidents with accumulative total.
For union sub-query, you need a label field as explained in the document. Then create similar fields\calculated fields in sub-query too and map them. (I am still unclear about exact requirement for sub-query)