No problem on the return. Bags will be checked through to MAN at MCO and you won't see them again until you arrive. (Maybe not even then <img src="smile.gif" border="0"> ).. .You may have to change concourses at ATL but they are all airside of security. Connected by train.
Outbound could possibly be a problem. You'll have to go through immigration and customs at ATL, put your bags on the 'connection' belt and maybe change concourses. Hour and a half is a bit tight IMO, but as you say there are lots of southbounds and Delta will put you on the next available if you miss the scheduled.