Error message 

ERROR #5001: element 'sql' is not allowed for content model '(annotation?,((true,false)|(false,true)|(true)|(false)))'

Indicates the <sql> element in the DTL XData block is in an unexpected location in the xml document, and that expected elements in that location are <annotation>, <true> or <false>. Maybe it is directly under <if> ?

Congratulations everyone, and thank you so much for making such an excellent community of developers possible !💐

You can query the message body class table with a left join to Ens.MessageHeader to get the orphaned message body identifiers :

select 
  %NOLOCK bod.Id
from
  MySample.BodyTable bod
  left join Ens.MessageHeader hdr on bod.Id=hdr.MessageBodyId and hdr.MessageBodyClassName='MySample.BodyTable'
where
 hdr.MessageBodyId is NULL