early 2015 Mac book software update issue

Usually when both the built-in Keyboard & Trackpad fail at the same time it is due to a hardware issue of some sort.....usually due to a bad Trackpad Flex Cable. This is a common problem specifically with the 2015 models (both the 13" & 15" models).

Also, with these 2015 models, both Keyboard & Trackpad communicate using two different methods. One is a basic USB communication while the system firmware is in control which occurs up until the login screen. Once you are authenticated, then the communication protocol changes to SPI.

If you are getting two different login screens, then that usually occurs because FileVault is activated and the user password is out of sync with the FileVault password. Once you authenticate at the first login screen, then macOS is taking over control from the system firmware so Keyboard & Trackpad communication transitions to SPI.

A bad Trackpad Flex Cable can also be a very intermittent issue. It took me a long time to figure out the cable was the actual problem which disappeared once I began replacing this cable. Occasionally the Trackpad, Keyboard, or even the Logic Board could be at fault, but this is rare unless liquid damage is involved. 2014 models are not affected by this issue, nor are 2016 models.

iFixIt is likely to have the necessary part, but you should consider professional installation since it is easy to accidentally damage internal components.

<< All the icons in the corner and the update complete pop up. >>

if you are getting a popup telling you to complete the update, your update got lost along the way and did not complete properly.

<< I’d assume the only way to fix it is to return to the previous version of Monterey before the update but I’m not sure how to do that. >>

To begin, you want to attempt to re-install the SAME version of macOS that is currently running, whatever that is. Since macOS never modifies itself, MacOS the same version as currently running can almost always be re-installed in place, right over the existing instance of macOS. By design, no user directories are modified during this process.

You use Recovery to accomplish this:

How to reinstall macOS - Apple Support

<< I’d assume the only way to fix it is to return to the previous version of Monterey before the update but I’m not sure how to do that. >>

You can never directly install Older over Newer MacOS, and it refuses to do so.

The way to make progress in that case is to use your Trusted local, disk-based BACKUP that Apple has been urging you to make at every turn. Since copies of your files are safely on the backup, you can ERASE your macOS volume and Install a different version, including older versions.

Once completed, you Migrate your files from your Trusted Backup using Migration Assistant, or at first run, with Setup Assistant (which uses the same process).