It could be more than half, it could be less. So the duration of the collision is not guaranteed to be half that of the elastic collision. The material which the ball is made of must be different, otherwise it would behave the same way (ie elastically). The reason is that it is a completely different situation.
It is impossible to deduce from the average force in a perfectly elastic collision what the average force would be in a perfectly inelastic collision with the same mass of ball and initial velocity. In the elastic case there is elastic energy stored in the deformed ball which is waiting to be released in the inelastic case there is no stored energy - it has already been dissipated. In both cases the ball has stopped moving, but in the inelastic collision it has stopped moving permanently, whereas in the elastic case it has stopped moving temporarily. However, the half-way stage in a perfectly elastic collision is not the same as a perfectly inelastic collision. But the ratio is the same, so the average force is the same. The change in velocity is only 5m/s and the time is 0.1s, compared with 10m/s and 0.2s. If you calculate the average force when the velocity of the ball becomes zero(ie after 0.1s), and when the ball has rebounded and left contact with the wall again (after 0.2s), then you will get the same answer of 100N.
When you calculate the force on the ball using the change in momentum of the ball and the time taken, you have calculated the average force.
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