First, the flats are only on the left of the corridor if you are walking down the corridor in a specific direction. If you turn around and walk back, they're on the right.
'Cornermost' is not a common word. From your description, Tom would appear to live in one of the end flats, but which one would be a guess. If there was an entrance to the corridor at only one end, Tom would probably live in the endmost flat at that end of the corridor.
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