I don't think a panel will be very interested in year 2. This sort of thing probably only happens when parents say "We've had ongoing extenuating circumstances for the past three years". A panel might then be interested to see how the child was performing before
all the problems started. Another reason for going back to the early years might be if there is a lack of recent evidence.
I agree with Sally-Anne that it's worth having the information with you, just in case anyone asks "How did she get on in an American school?"
I've made a brief comment about American English in the Q&As - section B32:
http://www.elevenplusexams.co.uk/11plus ... nswers.php
Obviously, the longer the child has been abroad, the more relevant this might be.