For question No.1, I wish I knew, but, that's one of the purposes of this forum, to try to find some answers
For question No.2, 117.GvIAP was at Crvena Crkva at the time for sure.
For question No.3 I think that at one point someone (possibly Igor Seidov) connected Bell and Shadrin, it was published and the story got a hold.
For question No.4 I can only assume, since general Andey Vitruk and his staff knew well what happened, and no one on US side knew anything about it, they decided to keep silent and it was like it until Shadrin told his wife about it and she said that to Igor Seidov, who was first to publish this story (as far as I know). Speaking of the 7 November 1944 incident over Niš, Red Army suffered huge casualties at the hands of 82.FG and it is understandable why the Soviets raised the matter so high.
For question No.5, they simply might have not known that they should have looked for Houghton near Bela Crkva. They interviewed thoroughly the locals, but the 14 October 1944 incident was a Soviet matter. At the time, Yugoslav (Partisan) authorities in the area were almost non-existent and the Red Army was out of any Yugoslav jurisdiction anyway. Maybe it is connected with question No.4 and all the evidence could have been removed.