First of all, welcome to the Forum.
Your post was very interesting, informative, and very useful to the unraveling of this story and puzzle. I have to run, so I cannot comment in any detail now, but will want to.
There is a young German university student (perhaps he has graduated; he was still in school a few years ago) who sells many regimental histories on CDs or DVDs for 5 Euros, generally, and I have bought a copy of IR 155 from him several years ago, for 5 Euros and modest postage. At the time he could send three for the postage of a single. These histories are scarce and expensive so this is a great service to the student of WK I. (I am not sure if I should mention his name, as it might seem commercial.)
IR 155 was a first-rate unit and included many ethnic Poles, as you can see from the death roll of the regiment. The other German units involved were inferior in several senses.
If you don't or can't get the CD I can probably transfer the relevant pages to you, but I will have to experiment a bit with the technology.
My Forum mailbox is probably full, but I could PM you my e-address.
I would be very interested in your detailed comments about the affair.
To add to my previous remarks, I have to say that my argument does not exclude the possibility that there indeed were some francs-tirerurs or hidden Russian soldiers in the town, but there is no proof save for the German records of troops being fired on. However, if there really were Russians attacking, what would be the point of such an attack? As the reminiscences of Boris Shaposhnikov [future Marshal of the Soviet Union, who then was an cavalry officer, 7th Cav Div, IIRC] say, the Russians were concerned with evacuating the draftees from the area and were giving the entire Vistula Curve up as planned (for years the Army was opposed to building roads and railroads in the are in order to make the Vistula Curve a good buffer zone).
The only reason of such an action one might think of is to provoke a retaliation on civilians and gain the propaganda profit. But who could really forsee that the Germans would indeed react with violent retaliation, with shooting hostages and shelling towns? And if it indeed was a planned provocation, why did it happen only in two places and only at the very beginnig of the war? if there really were some Russian "agents-provocateurs", the events in Kalisz would be a proof that it really works. So why there were no more such events? It's not that the Germans have become more lenient - in each captured place they were routinely issuing proclamations that any hostilities agains the German military would be dealt with severely, including capital punishment.
The troops of the IR 155 were indeed first rate, yet it has to be remembered that even the best peacetime training cannot substitute for the real thing, cannot train people to overcome fear and to distinguish between a stray shot somewhere and a fussilade aimed directly at them.
All the above convinces me that the reason of the Kalisz event was the lack of combat experience of the troops resulted in "trigger happines" and "friendly fire". Coupled with the pre-war fears of armed "francs-tirreurs" the results were gruesome. However, I would gladly change my opinion, should any historian come up with new evidence (which, if exists, IMHO could be found only in Russian archives).
This discussion has inspired me to think about writing an article in English about the whole affair and publish it somewhere. "War in History", perhaps?
The collection of the German student is known to me, but when I ordered a large amount of regimental histories, I did not include the IR 155; I do not remember why. Either I wanted to cut back on costs, or it might have not been in his collection yet. Is he still selling them?
Your mailbox seems indeed full, so if you wiuld be so kind as to sending me your e-mail, I would be grateful.