Probably quicker, in fact, to use the verb "Complete" which is provided in the Omnifocus applescript dictionary, and which will find nested contexts directly.
Code:
tell application "OmniFocus"
tell first document
set oTask to make new inbox task with properties {name:"Testing"}
try
set context of oTask to my FindContext("Email")
end try
end tell
end tell
on FindContext(strContext)
tell application "OmniFocus"
tell first document
set oContextList to complete strContext as context maximum matches 1
if (oContextList is {}) then
else
return context id (id of item 1 of oContextList)
end if
end tell
end tell
end FindContext