By a coincidence I discovered that I need to do guid:sub(6, 10) (and not 7) to encompass all the creature ID bits. I checked with some other addons and noticed that Recount is using 7 too in at least one place.
I don't have an extracted copy of FrameXML with me at the moment, but I could have sworn they'd added some global constants to define those bitmasks/sizes.
*) The deletion popup mid-scenario when members join or leave the group.
This part seems to be working now, thanks!
I had hacked around this locally by adding a line at the start of Recount:ShowReset() to test if any fight data had been stored, and if not, then return early. It didn't do anything to help the "mid-scenario" situation specifically, but it would avoid repeated popups when nothing had happened in between.
Are there a Warlords-beta version of this addon incomming? Seems blizzard has enabled addons now, and Bugsack and !Buggrabber works without error (well, all I did to them was to change the interface number to 60000 in thier .toc files).