"I think a bigger thing was made of the pyramid in the couple of years following Russell's death by Edgar Morton, Woodworth and Fisher than Russell actually did during his lifetime."
Yes, I agree. Russell only wrote about the pyramid in a couple of article published in the Watchtower, in chapter 10 of volume 3 of Studies in the Scriptures ("Thy Kingdom Come").
So while he might have believed that the Giza pyramid was "God's stone witness", Russell really did not promote that belief.
There are indications that the numbers used by Russell in the pyramid calculations in Vol 3 could not be rigourously documented, if that is so, then Russell probably did not want to bring too much attention to the subject for fear that someone would raised questions about where exactly did the measurment numbers come from.
Someone DID raise the question in 1904, and Russell answered in the Watchtower saying that he saw no reason to believe the numbers in SiS3 were inaccurate.The following year (1905) Russell changed the pyramid numbers in question for all future editions of SiS3.
As you mentioned, the promotion of the significance of the Giza pyramid was done by others. In particular by Morton Edgar, who wrote many books on the topic, and was particularly upset when Rutherford disposed of the idea that the pyramid had any significance in two Watchtower articles published I believe in 1927 or 1928 (I am still trying to find those articles).
Judge Rutherford called the Giza pyramid "Satan's Bible"
Morton Edgar called Judge Rutherford "this evil servant"
Morton probably discontinued association soon afterwards.