Firstly - please excuse me if my earlier post, which suggested that A,C,D might be on the same rank or file of the playfair square, has misled. At that time I'd not looked at the puzzle for long and certainly hadn't considered all the possibilities.
Dryden, it's certainly not with enmity that I disagree with you ("Without that process of deduction, the puzzle has no point, hence my plea above"). Firstly, I think that the puzzle is fundamentally flawed if it cannot be progressively solved, instead requiring a big bang. Secondly, it goes against my belief to withhold information, if it can be useful to others. Thirdly, and perhaps the clincher, is that there are people on another site crowing that John Green will have a sparse mailbag for this puzzle. If in doubt, I will do what such people least want me to do. So anyone who doesn't want to know the keyword, look away now. In reverse : sex, obgnik & cap