In so far as it isn't clear yet (to other readers), I think this image may clarify why you set AOB to 90 degrees when looking to the target, and use ownspeed*sin(constant_bearing) as target speed.
As far as the TDC is concerned when the bearing is fixed to the target, to know the torpedo lead-angle it only needs to know what that sideways speed is perpendicular to the target bearing. Hence why you set the AOB to 90, and target speed to ownspeed*sin(constant_bearing). The TDC will calculate the lead angle for the torpedo by taking a right-angled triangle with that sideways-speed as opposing side, and the torpedo speed as the hypotenusa. So torpedo lead-angle will then be:
arcsin( ownspeed * sin(constant_bearing) / torpedospeed )
And then further corrections for gyro-angle parallax based on (rough) range.
The real target speed and actual AOB isn't so important to get the torpedo on the target as it will only provide additional information on the speed components along the line of bearing. This will only tell the closure rate of the torpedo to the target, Read: correct time to impact. The time to impact calculated by the TDC cannot be relied upon in this Ausdampfverfahren setup. Without knowing the actual AOB and speed, the torpedo would be lead onto the target anyway. (assuming measurements are exact without error) The idea behind the Ausdamphverfahren is to fire as soon as this constant bearing is established and the calculation of the sideways speed is done. Or whenever you got up to a sufficiently close range.
If you want to fire on a different bearing to another target (than the one on the established constant bearing) at a later point. Then you need to have the scope on the established constant bearing, correct target speed to the real target speed based on the observed AOB (targetspeed = 'sideways speed' / sin(observed_AOB) ) and set the observed AOB in the TDC. Now the TDC is calibrated to aim wherever with proper torpedo lead. (As long as range is entered appropriate to the target) This was explained in derStosstrupp's documents too iirc.