I've asked Arnie to try to answer the specific behavior on the merge files.
I didn't work with them much. There is probably still room for improvement if we can decide on a desirable feature set.
The release notes have been collected from their original site and moved to
(thanks Sam :-) )
These may not have all of the information you want about changing versions but it's a start. It's been 4 or more years since I've used webwork 1.9 so my memories of its exact behavior are starting to get fuzzy. :-)
Hope this helps.