Hosting Monterey, Mountain Lion, and Mojave in an Intel Mini

Back in 2022 I purchased the final Intel Mac mini model, planning to run Parallels 17 under MacOS Monterey to save apps that run under Mountain Lion (10.8.5) and Mojave (10.14.6). This I did, but the apps I care about the most won't run in the virtual environment. So, I just bought a 4 TB SSD drive, created a HFS+ partition for Mountain Lion, downloaded the installer .dmg file from Apple, and have tried to use it to install Mountain Lion in the HFS+ partition on the SSD, but no matter what vintage of computer I attach the SSD to, I get the same messages when I double-click on the .pkg file: (1) The certificate is expired. (2) Do you want to proceed anyway? Yes. (3) "OS X Mountain Lion is not compatible with this computer". The behavior is exactly the same on an older Intel mini running Snow Leopard, or a 27" iMac running Mojave. All three see the HFS+ partition/volume I named "Mountain Lion". If you think the issue is the expired certificate, I do still have a (2012?) installation USB for Mountain Lion.