It's worth pointing out that there are other instances of rules that don't say what they really mean, so putting too much emphasis on making any rulings consistent with the above statement in the rulebook might not necessarily be the best approach, especially if it starts to produce results that are counterintuitive.
The card itself says "Play this as..." So you are playing it.
So... It only says "Play this as..." on the first play?
TR: Choose BoM
1. BoM(+A): Choose CardX(+B)
2. BoM(+A): Must Choose CardX(+B)
Vs. "This is..." on the second play...
TR: Choose BoM(+A): Choose CardX
1. CardX(+B)
2. CardX(+B)
Yeah it's either:
TR: Choose BoM(+A): Choose CardX
1. CardX(+B)
2. CardX(+B)
Or:
TR: Choose BoM(+A): Choose CardX
1. CardX(+B), trashes self
2.1. BoM(+A): Choose CardY
2.2. CardY(+C)
I have to confess that I had been having real trouble reconciling the way the rulings on this subject had gone with my mental picture of the way playing a card worked. In particular I was having trouble with the idea that playing a Band of Misfits amounted to playing two Actions. The problem was that I was thinking that the player made the choice of what card BoM was to be before actually playing BoM. Overnight I've realised why that was wrong: one follows the instructions on a card as part of playing it. Therefore the instruction "Play this as if it were an Action card in the Supply costing less than it that you choose" can only take effect as part of playing BoM, so there are indeed two Actions played, namely BoM and then the transformed card.
Having now corrected my mental image of playing BoM, I've been able to answer a hypothetical question I'd been pondering ever since this discussion started, namely "If there were a card, Band of Confusion say, that said '+1 Action. Now play this as if it were an Action card in the Supply costing less than it that you choose. This is that card until it leaves play', how would it work?" Exactly as BoM with the +1 Action token on its pile.
So, I'm now happy with the way the rulings have gone, something of absolutely no consequence to anyone else in the world but of great importance to me as it means I can once again understand BoM by applying logical rules.