Given that your throttle went to WOT on a Preset Failsafe Test, so to speak, that suggests that you set the preset failsafe (or never set it in the first place) and then reversed the throttle setting on the transmitter w/o rebinding in the Preset Failsafe with the new throttle setting. That is kind of easy to forget.Have you tested the servos using a programmer/tester or another Rxer? I have plugged servos in backwards before without damaging them. But I have never done that with two on a Y. That could have doubled the load typically seen when plugging them in backwards. But, I guess I would have to see them on a tester to be convinced they were bad.
I suppose it is possible that you could havehosedup the Rxer with two servos on a Y and plugging them in backwards. But, I don't think that would mess up your binding capability. Have you measured the Voltage on the Rxer's buss and the current on the AILE circuit when operating a servo. If that is O.K., then I am not sure why the Binding on Preset Failsafe would be affected.
I have found that when binding in the Preset Failsafe settings, I have to hold the bind button a bit longer than when just binding. Don't ask me why. But it is this way on both my DX8s. For the Preset Failsafe to take, I end up holding the button a few seconds after the power button blinks on the transmitter. There can be a lot of froggin' around when doing the preset failsafe. Forget the control surface settings and set the throttle to idle and just try to get the bind for the preset. Then once you got the timing down, then do a preset failsafe bind adding in your control surface settings.