How to fix Permission Denied error when starting up MongoDB
Use homebrew to install mongodb so you don’t need root would be my help to offer. 


https://brew.sh


Then 

brew install mongodb
brew services start mongodb 




As to sudoers, boot into recovery and use terminal (utilities menu) to mount the filesystem and fix it per this answer from your linked question. Older os can use single user mode, but terminal in recovery is nicer to use.