Open farthest right tab in Safari
That was a fun question to tackle! Follow the instructions below and paste this code in step 1.8:

set n to 0

tell application "Safari"
    repeat with w in windows
        try
            set n to n + (count tabs of w)
        end try
    end repeat
end tell



tell front window of application "Safari"
    set current tab to tab n
end tell


This will tell Safari to change to the right most tab. Then, to map it to a key shortcut, just follow the steps listed here by Monomeeth which is shown below. The code was adapted from here

In summary you will need to use:


Automator to create a service that runs an Apple Script
System Preferences to assign a keyboard shortcut to the service


Below are the steps involved for each. 

1. Create a service using Automator


Launch Automator (usually found within your Applications folder)
Go to File > New
Select Service and click Choose
In the top right hand of the window that appears, ensure that "No input" is selected from the Service receives drop-down list
Ensure the In drop-down list is showing "Any application"
In the second column containing the long list, find "Run AppleScript" and double-click on it
This will add the Run AppleScript window on the right
Now, replace the (* Your script goes here *) with the script you want (the one I provided above)
Save the service using a meaningful name (e.g. RightMostTab).


Now to the next step.

2. Creating your shortcut


Go to System Preferences > Keyboard > Shortcuts
Select Services from the sidebar
Find your service (it'll be in the list on the right-hand side)
Add a shortcut by double clicking on the service name
Now go to System Preferences > Security &amp; Privacy > Privacy
Select Accessibility in the sidebar
Click on the + sign (you may need to unlock the padlock if it's locked)
Add Automator.
Add Finder (to find this you will need to navigate to /System/Library/CoreServices/Finder.app).
Exit your System Preferences.


Let me know how it goes!