Apple Hardware Test Error Code 4MEM/9/4000000: 082525818

2) how can I stop the update from installing without restoring from a Time Machine backup or reinstalling from the USB drive?

Unfortunately, your non-disclosure agreement and the ToU here prevent us from being able to discuss anything beta related, so we cannot help with anything but the RAM issue which could be failed RAM, incorrectly seated modules, etc. You can either take it to a Genius Bar for another test and/or replace the modules.

That error is used to report a RAM Memory error.

There are more than 50 different models.

Exactly which Model MacBook Pro, and which size screen?

Your Mac supports exactly what you have in it, two 8GB modules, one in each of its two slots.

You should not be running Faster or Slower modules in your very fussy MacBook Pro other than these:

RAM Type:

PC3-10600 DDR3

Min. RAM Speed:

1333 MHz

Details:

Supports 1333 MHz PC3-10600 DDR3 SDRAM.

from:

https://everymac.com/systems/apple/macbook_pro/specs/macbook-pro-core-i7-2.2-15- early-2011-unibody-thunderbolt-specs.htm…

If you have modules with a Vendor sticker on them, they may have a lifetime warranty. Contact the Vendor for replacements. Their sticker is the only "proof of purchase" you should need. (Advance Replacement may be available if you guarantee the return of the old modules with a credit card number.)