[AJames]

The questions you pose are interesting.

Firstly, you are correct in saying that John Herschel did not name NGC 4755 as the Jewel Box. It seems that its origin of the name was made in some causual quip at one of the London Astronomical Society (Now the Royal Astronomical Society) meetings in the late 1830's.
Like any given name or nickname, the origin of it remains in question, and the person who stated it is often lost in the confusion. It is probably like saying, who was the originator of the joke; "Why did the chicken cross the road?" The person continues to be nameless, even though it is probably quoted as a common joke through the known world.

I also think the Jewel Box does not necessarily mean just coloured stars. Ie.
You can have a Jewel Box just with diamonds you know! (A any decent female would probably desire!) Although perhaps inaccurate, I think it at last portrays the lustre of a small collection of brilliant stars in a small area.

Oh! ... and Merry Christmas to you all!

Andrew

Note: Your timing could not have been better. I have written an extensive article on the Jewel Box, which I was going to post on the 1st January, but due to your post here, I have posted it tonight. This was an article written almost 10 years ago, which I have updated in recent weeks.
In Southern Astronomical Delights Pages, the Jewel Box article, the Index starts at; http://homepage.mac.com/andjames/Page001.htm

The portion written on the historical aspects you describe can be read at;
http://homepage.mac.com/andjames/Page003.htm
and
http://homepage.mac.com/andjames/Page004.htm

Note 2: I have place some drawn colour images of these "colourful" open clusters in another Southern Astronomical Delights page, which highlight your point here; Ie. See
http://homepage.mac.com/andjames/Page03002.htm and
http://homepage.mac.com/andjames/Page03003.htm

Note 3: It was H.C. Russell - not W.C. Russell, which heads the 1872 article.
I have a soon published paper on this reference examining the actual micrometrical measures and the alleged changes in NGC 4755 based on the current positions.