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Question: This was posted half a day ago and was estimated to happen (or not) in less than 1.5 days from now. Now, or later today, when there is less time to the collision, can't they calculate with far more accuracy, whether the objects will collide?

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Only if one of the two objects has passed over one of their tracker stations (and they got a good scan of it)

a 0.001% uncertainty in speed is about 7km uncertainty in position after a day

Then how can they say 1-20% risk of collision?

prob they can measure speed at a higher accuracy than one mm/s, also orbit have a relatively low inclination difference and objects have a size, so that the impact area is not a point but a larger ellipses of varying overlap between bodies.

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