Great post and great song.
You are 100% correct on everything, except one small detail ... Gilmour was originally brought into the group to play guitar while Syd tried to sing and play a bit of guitar, due to his Acid addiction (??). Gilmour did not initially replace Syd. There was a period of almost 2 years and one album (if I recall correctly) where both were full members of the band. Syd more, or less, just drifted away. I don't recall any grand announcement, or otherwise, of his departure. The remaining members loved Syd ... the Wish You Were Here album is dedicated to and is about Syd.