Bryce - I think your pic of Queen Mary and page 211 of Roberts show one and the same chain-link choker and with the brooch centre front. No doubt about it. I do think the photo in Roberts is disappointing. (I found the photo of Queen Mary's Love Trophy collar extremely disappointing too.) Earlier dating of the Bassano/Elliott & Fry portrait is not a problem. It still predates the chain-link choker.
Edit: I would just add that not all the descriptions used for the Royal Collection online items have been correct. There have been some bad errors. I would suggest that the writer of fthe description was using "chain-link" from Field. More than one publication from the Royal Collection has relied quite a bit on Field.