To reinstall Safari 11.1.2 over Safari 12 you must first disable SIP (System Integrity Protection) using csrutil in terminal. When SIP is on the system will not let you overwrite any needed system files ... Safari and its other system file are SIP protected files.
Here's how I reinstalled Safari 11.1.2 on High Sierra:
Download Pacifist
Download Safari 11.1.2
Restart your Mac, holding down ⌘-R
Open terminal from the Utilities menu
Type: csrutil disable
Hit return
You'll see a message that SIP is disabled.
Type: reboot
Hit return
Your computer will reboot
Launch Pacifist
Click the Open Package button
Open the Safari 11.1.2 package
You'll see the contents & subfolders listed
Select the line Contents Of Safari 11.1.2 Sierra.pkg
Click the install button.
Once you do, you will get a few dialog boxes. You want to install with admin privileges, and you want it to overwrite anything it asks you to. So check the "do this for all" checkboxes in any of the dialogs that come up. Otherwise you'll be clicking Okay dialogs forever.
Once it says the install is complete quit Pacifist & launch Safari. If you didn't touch your previous extensions, they should all be there and working. If they are not, find old installs of the older extensions and reinstall them. Remember if you select Safari extensions from and old backup, you have to double click the extensions to reinstall them. You may be required to install them from the (old) Apple Extensions Library ... but that's better than having to use the new "App Extensions" Apple is forcing us all to use.
One other thing you need to do is turn off "Automatically check for updates" you have set in the Mac App Store preferences. Otherwise you will get nagged to update to Safari 12 again.
If you want to turn SIP back on to protect your system again:
Restart your Mac, holding down ⌘-R
Open terminal from the Utilities menu
Type: csrutil enable
Hit return
You'll see a message that SIP is enabled.
Type: reboot
Hit return
Your computer will reboot
Done! Enjoy old Safari goodness!
_____
I reinstalled AdBlock and deleted AdBlock Plus. AdBlock Plus now throws up a page saying how everything is changing with Safari 12 blah blah blah. It may be temporary .... BUT IT IS ANNOYING! So I reverted back to and old version of AdBlock (which is different (and use to be better) than AdBlock Plus) I had in a backup. If you do the same & have custom filters you've made in AdBlock Plus be sure to copy them and add them to the custom filter list in AdBlock before you delete AdBlock Plus.
Did a little digging (after posting this) and found the self-hosted version of
AdBlock direct from the dev's website. In my book ... it's the best by far! Hopefully they will be able to port many/more of the features to the new "app extension". My fave is the ability to block an element from a website via macOS contextual menu.
I'll be waiting quite a while before I update any of my computers to Mojave. I've already seen extension writers that have no plans to learn how to write an Apple app and have to work within the restrictions Apple is placing on system access.
Okay .... now I'm done.