by "Sophia"
(188.8.131.52) on Tue 28 Sep 2021 23:56:53
public update: I was able to find the source of the problem and solve it. All of the downloads are fixed now (same software contained in the DMG files, but the DMG process was redone to support older systems).
for the sake of anyone coming upon this thread and wondering what the problem and solution were: The command-line tool hdiutil as of macOS 11 / Big Sur by default creates DMG files APFS as the file system, whereas prior to 11 the default was HFS+. Mounting APFS requires macOS 10.13 or higher. The solution is to add to the command-line arguments: -fs HFS+