download Catalina without updating

I tried installing Catalina on my MacBook Pro and ended up with a loss of the entire file system, making it unbootable. It got hung up during the install for over 12 hours, and so I powered down. When I tried starting back up, all I got was the blinking folder icon with the ? on it. Unable to boot into recovery mode. I have a backup of the files, but not a bootable drive. In the meantime, I have updated my late 2014 iMac to Catalina, and I am unable to download the Catalina Install app because once the download finishes and the update tries to start automatically, I get an error message that just says that it encountered an error and is unable to continue. Before hitting the OK button, I searched the entire disk for the Install App and could not find it, suggesting it was automatically removed already. Hit OK, and I get the message that my Mac is up to date with Catalina.

I have booted into Recovery mode and all disks appear to be OK. I thought that I might go back to Mojave, and I have Mojave Time Machine backups, but so far, Catalina has not been able to do any Time Machine back ups of Catalina. In add-on, in trying to download a copy of Mojave to create a bootable Mojave disk, I get an error message as well.

In the meantime, Mail keeps quitting with the message "Mail cannot save information about your mailboxes because there isn't enough space in your home folder. Quit Mail and delete any files you don't need. Then open Mail again." My home directory has 157 GB free space in it.

And, related, I fear, is this nonstop message in the console:

error 11:22:34.758609-0500 mdworker_shared Messages Importer couldn't import <private>

error 11:22:35.624701-0500 deleted unable to create CacheDeleteDaemonVolume for <private>

error 11:22:35.625094-0500 deleted unable to create CacheDeleteDaemonVolume for <private>

error 11:22:35.733915-0500 deleted fs_snapshot_list failed: Operation not supported

error 11:22:35.733941-0500 deleted Failed to enumerate snapshots for volume <private>: Operation not supported

error 11:22:35.735523-0500 deleted_helper APFSIOC_GET_PURGEABLE_STATS: Inappropriate ioctl for device

error 11:22:35.747363-0500 deleted fs_snapshot_list failed: Operation not supported

error 11:22:35.747482-0500 deleted Failed to enumerate snapshots for volume <private>: Operation not supported

error 11:22:35.749393-0500 deleted_helper APFSIOC_GET_PURGEABLE_STATS: Inappropriate ioctl for device

error 11:22:35.760065-0500 deleted fsctl error: Inappropriate ioctl for device, using HARDCODED desired threshold 12335448064 for volume_size: 15653560320

error 11:22:35.760113-0500 deleted fsctl error: Inappropriate ioctl for device, using HARDCODED low disk threshold 10737418240 for volume_size: 15653560320

The Mail app issue may have been related to using SpamSieve with it. It is supposed to be compatible, but perhaps not yet?

The copy of the installer was not valid. Looks like an incomplete download.

Here is the message i just got twice in a row after trying to download the installer again:

It is not on the computer. I searched via the Terminal as well.

Looks like the low disk threshold message was for a 16 GB USB flash drive. I ejected it and messages stopped.

OMG, all of a sudden I have a copy of Install macOS Catalina.ap in Applications. This time when I went to the App Store, the Get button was replaced with Install.