1) The result is very highly statistically significant. They go into great length showing this.
2) They do post the full list of names. Again, a quick read would tell you this information. Boston and Chicago are both major U.S. cities, if you weren't aware. The fact that institutional racism is present in them is therefore obvious important and meaningful. This study has been replicated many times since then in other settings as well. Besides, you claim that the fact that discrimination against blacks isn't important because it is in a city with a high black population, but that is totally backwards.
3) They sent these to all the job ads posted in the local newspapers, thats a pretty reasonable set of firms, and a very large one as well.
4-5) And the conclusions are not based on conjecture, literally the only thing different between the two resumes is the name. Everything else is controlled for. This is therefore a perfect controlled experiment to see if the name is the thing causing discrimination. And since the names are carefully chosen based on racial representation, these studies provide incontrovertible evidence of widespread racial discrimination.