In this case, since EAMES already works for COBB, I think the meaning is more along the lines of definition 5 or 5.1 at oxford dictionary.com's ON definition (thanks to MaulikV for the link):
(Of an actor) on stage; performing.
That's your cue. You're on.
(Of an employee) working; on duty.
I would lean more toward the "actor giving a performance" definition, as COBB's task is to trick someone into giving away information, but I think both senses are appropriate.
In general you are correct that stay away and keep away can be used interchangeably without loss of understanding, meaning don't go near. Stay away seems more AmE while keep away seems more BrE.
One way of thinking about the distances is to see usage of the keywords in other related phrases. As an native AmE speaker, my thinking behind my sense of the differences in distances involved is
far from is the most distance, as it is indeterminate
far from us, another civilisation may exist
keep far away is further away than keep away
keep away is the closest, keep also has the meaning of possession which can imply things being at-hand or within reach
keep clear of the moving train
keep to yourself
stay away is somewhere in-between and possibly inclusive of far away's distance
He pointed and said "stay over there".
One would never say keep over there, thereby giving stay a farther meaning.
Stay far away and keep far away are interchangeable for me since the distances inherit the magnitude of far
For stay away, keep away, and far away the quantities involved are not as easily defined as the other descriptive adjectives a few, some, or several
Best Answer
A decade means a time period of 10 years. You could say something like:
Now this is 2016, you actually quit the job in 2009. Technically it was just 7 years ago. But you can still say it that way. But you cannot refer '2003-2010' as a decade because that's only 7 years. However, say you were a vegetarian and you quit meat in 2003 and that went on till 2010. Now, you could say that:
That doesn't mean you were a vegetarian for 10 years. So that is okay. But you cannot refer 7 years as a decade. Hope this helped.