In the context of the article your first point is the key question. The author argues that the flow rate is relatively constant even with varying speed. I would tend to agree that it would be fairly constant but if there was extremely efficient merging I could see the flow rate increasing by a few percent. Theoretically the difference in flow rate if cars maintain a 2 second delay from the previous car should be the difference in the length of time that it takes the actual length of the car to pass through the merge point.

So if the car is travelling at .5 carlengths per second then the duration per car should be 4 seconds whereas if the car was travelling at 10 carlengths per second the duration per car should be only 2.1 seconds.