Disk Utility not open DMG of corrupt boot drive
I have 3 Macbook Pros from about mid 2007- early 2008 with pretty much identical hardware. Recently one of them would not boot- would slowly boot into recovery environment. I swapped the HDD into all the macs and the behavior was the same. I do not have a recent Time Machine backup of this drive yet have some critical files on it- yeah I suck. I was able to make a DMG of the entire drive using the recovery option in disk utility. I am now VERY KEEN to restore this DMG but have come up with every conceivable error the latest being something like "no mountable files found". I realized I am trying to open/repair this DMG in High Sierra using APFS and it was likely made in Sierra on an Extended Journaled file system. I don't know what is relevant so please wade through this and let me know if I need to add more info or if it's irrelevant. However I cannot open/ repair it in an old desktop version of Mountain Lion.
I am desperate to make this a bootable disk with all files and programs intact. I understand that as the original drive was corrupt this may not be possible. I have tried command lines (pretty much a novice) but have had no success. I am happy to buy a third party app to help this happen. Any ideas very welcome!