Yes, I do think that Bloody Sunday was accurately estimated by historians. There are some people who spend their entire lives making careers of studying things like this, and they are really good at their job. They would be able to make a pretty close guess of how many were killed.
How can an accurate estimate of injuries or deaths be obtained, past attempts have shown that in the heat of battle or after a disaster of any kind people are confused, angry and generally unable to have a clear head, which is imperative when trying to ascertain the resulting damage done to property as well as people. The human race as a whole, when faced with loss of lives and terrible destruction will either underestimate so as not to make an already bad situation worse or over estimate in order to force some sort of response from the public.
Historians on both sides of the issue have probably overstated and underestimated the casualties of the Bromberg Bloody Sunday incident. The accurate count is probably somewhere in between all of the estimates. The number changes depending upon whose dogma is trying to satisfy an end. Now that World War II is 70 years in the past, perhaps a more accurate and exhaustive account can be done.
Its always so hard for people to ever accurately figure out how many people died in a conflict or an atrocity, as people in those situations don't tend to keep fantastic records, and the chaos of the moment makes things hard then. Estimates work, often, but they aren't always reliable.
No, I do not think that the number of casualties during Bromberg Bloody Sunday has been accurately estimated by historians, because the Germans wanted to inflate the number for political purposes. The Germans had the power to do such a thing at the time, as they tightly controlled what information was released. The Germans wanted to use the incident to justify hurting those in Poland deemed against the state.