For clarity, the whistlers are the black tipped darts with a little rectangular hole on the side to release air from the internal hollow. I believe these were discontinued somewhere around 2009, as we saw the N-Strike series transition towards pretty exclusively using streamline darts.
The dart tag darts are more accurately called tagger micro darts. They have the velcro tips, and small circular holes to release air from the internal hollow. There was a whistler and non whistler variety. These were discontinued by 2013, and so overlap with your date range.
With 100% confidence I can say that Nerf brand N-Strike Whistlers and Dart Tag darts were always hollow, no exceptions. The design intent behind them is for the hollow to act as a cushion, and I can't think of what sort of manufacturing process defect would lead to darts without that hollow.
There are many types of darts out there, but the other type with a hollow head are Streamline darts.
I bring them up because there are some 3rd party knock-offs that have totally solid tips. Some of these are soft plastic, some hard. They're fun for backyard shooting, but are not welcome at most nerf wars (certainly not any that I host).
But were the Whistlers and Dart Tags completely hollow from stem to tip all the way through? For my research there can't be a wall separating the hollow stem part from the hollow head part, it has to be hollow all the way through as on one completely hollow piece. By the way thank you for the information and links. That will help my research a great deal!