"If a quick lineout is being taken and an opposing player gets in line with the player taking the lineout, can it still be thrown backwards to one of his own team so he has no opportunity to contest it?"
First off we just need to differentiate between a 'Quick Throw-In' and a 'Quick Lineout'.
A Quick Throw-In is taken before a lineout has formed, and can be taken anywhere from the line of touch, to the throwing team's goal line. The ball can travel straight or backwards, as long as it goes over the 5m line. The usual constraints apply such as using the same ball, ball not touched by other players, etc.
A Quick Lineout is taken once a lineout has formed, at the line of touch, but before the other team is ready. As long as the lineout has formed, this is legal. A formed lineout is defined as two players from each side.
Alun, I think you are asking if an opposition player can contest the Quick Throw-In?
The answer is 'Yes', providing he does not encroach within the 5m channel. The thrower can still throw the ball backwards to his own player as a lineout has not been formed. Remember a lineout consists of two players from each side.
The Rugby Ref hopes that has answered Alun's question.