To get around the expired Cert you'll need to use a second Mac system to download the OS installer file and then setup a USB thumb drive as a bootable OS installer (don't forget to first format the USB drive to GUID-Journaled) Goto this cute to get the OS file [https://support.apple.com/en-us/102662|How to download and install macOS] then follow this guide [https://support.apple.com/en-us/101578|Create a bootable installer for macOS].
See if that gets you going, good luck! You may need a new drive.