Thanks for reporting and helping...
Well, about that "I want them all uncompressed in a folder" support.....you need to see how MAME originally works. A set usually contains more than one file (gfx roms, sound roms, dumps holding the code etc), so they got a container, a folder named after the set, holding these files. Using an archive (7z/rar/zip) is pretty the same - a container (again named after the set).
Scanning/Merging/Rebuilding is built around this.
I see your point when it comes to 1-file-per-set collections. Keeping them decompressed leads to the a bit obsolete storage method with the extra folder.
What you can do is to change the datfile definition to only define one set where all files are stored. This would allow to have all files in one folder (named after the setname defined in the dat). Cons: you loose additional information like a detailed description per set (however as you can see from these big filenames, they simply put all in there...) and you get the scan results after finisihing this one giant set.
Theoretically I can add an option which changes a loaded datfile into an one-set-billion-roms dat......time will tell