Plugin Menu Missing

You can install plugins manually: https://codex.wordpress.org/Managing_Plugins#Manual_Plugin_Installation

But, manually activating them (without a Plugins menu) would involve modifying the database, so you might as well just modify the database to take over the existing admin account instead.

First, you’ll need to find phpMyAdmin in your hosting account’s control panel. (consult your hosting provider’s documentation, or contact their support, for specifics)

Then, change the password on the existing admin’s account following this guide: https://codex.wordpress.org/Resetting_Your_Password#Through_phpMyAdmin