1. The x-axis in the plot by Amber.G is min:sec. If you look at the leftmost point, it is 19:55. At the next gradation, it becomes 20:00; and the next is 20:05. The only way this is possible with all the gradations on the x-axis being equally wide is if the xy in 19:xy is in seconds. (Added later: it also works if xy is in minutes...but that's well, less likely
2. I dont think the 2-second delay will contribute to an error (if anyone thinks it might) since the 2-second delay applies to all transmission uniformly. Now, if the time delay is non-uniform, it is a different story.
3. The plot did not clarify line-of-sight as per what- CY2 orbiter? Madrid (The station that could see CY2 at that time). If Madrid, then one has to know the angle of Madrid-to-Vikram landing spot to do a velocity calculation for estimating absolute magnitudes. Otherwise we are seeing a plot that is velocity*cos(90-angle) (per first order simplistic trig.). Need to draw it.
4. It was puzzling to see the velocity shoot up from 400 m/s to 1400 m/s just 12 seconds before it was braking. Not clear to me why that should be the case. Before it brakes, it is in orbit, and not sure if that rapid increase was due to change in orbital velocity- does it change that drastically, or did it change its attitude first, and then started braking. The other possiblity is that the line-of-sight of the receiving station changed at 1400 m/s due to......change of direction of the lander. This confused me, so I did not attempt any calcs from this plot.
5. I feel that the plot shown in the Mission Control seconds before and after the telemetry stopped might be the most accurate velocity data released, since (I assume) it came from the lander itself. That said, the scale was really large: 2 km per gradation of the plot. At that large least count, even a dot would be 10 m wide, so the error builds up.