[DamonC]

I've rewritten the script to meet your needs and tested it with my own contexts. I've gathered your @ contexts and actual contexts into lists at the beginning of the script with a third list that categorises them into top-level, sub-level and sub-sub-level contexts.

The script then gathers the actual contexts from OmniFocus and then tests the ShortContextList for the selected action in OmniFocus to determine the correct context to apply to it.

set TopContext to "URLs"
set SubSubContext1 to "Download"
set SubSubContext2 to "Watch"
set ShortContextList to {"@add2sys", "@blog", "@bm", "@dl-app", "@job", "@mail", "@notepad", "@optimism", "@parse", "@queue", "@read", "@rss", "@thinkabout", "@watch-long", "@watch-short"}
set FullContextList to {"Add to System", "Blog About", "Bookmark", "App", "Job Listing", "Mail to Someone", "Project Notepad", "Optimism", "Parse", "Queue (Add To)", "Read This", "RSS (Add To)", "Think About", "Long", "Short"}
set ContextTypeList to {"SubContext", "SubContext", "SubContext", "SubSubContext1", "SubContext", "SubContext", "SubContext", "SubContext", "SubContext", "SubContext", "SubContext", "SubContext", "SubContext", "SubSubContext2", "SubSubContext2"}
set OmniFocusContexts to {}
set i to 1

tell application "OmniFocus"
	tell front document
		-- Capture the top-level, sub-level and sub-sub-level contexts
		repeat with theContext in FullContextList
			if item i of ContextTypeList = "TopContext" then
				set the end of OmniFocusContexts to context TopContext
			else if item i of ContextTypeList = "SubContext" then
				set the end of OmniFocusContexts to context theContext of context TopContext
			else if item i of ContextTypeList = "SubSubContext1" then
				set the end of OmniFocusContexts to context theContext of context SubSubContext1 of context TopContext
			else if item i of ContextTypeList = "SubSubContext2" then
				set the end of OmniFocusContexts to context theContext of context SubSubContext2 of context TopContext
			end if
			set i to i + 1
		end repeat
		
		
		tell content of document window 1
			set theSelectedItems to value of every selected tree
			if ((count of theSelectedItems) < 1) then
				display alert "You must first select a single task." message "Select a single task before running this script." as warning
				return
			end if
			if ((count of theSelectedItems) > 1) then
				display alert "You must select only one task." message "Select a single task before running this script." as warning
				return
			end if
			set i to 1
			-- Test for the short context and apply the correct context to the action
			repeat with anItem in theSelectedItems
				repeat with AShortContext in ShortContextList
					if the name of anItem contains AShortContext then set context of anItem to item i of OmniFocusContexts
					set i to i + 1
				end repeat
			end repeat
		end tell
		
		tell document window 1
			set selected view mode identifier to "project"
			tell sidebar to select inbox
		end tell
		
	end tell
end tell