I think enthusiasts just never knew who these people were so some enthusiast just made up a name and everyone copied. This sort of thing is very common with announcers. Pauline Cavilla and Mark Stutter (the old Thales system announcers from 2005 to 2012 at Southeastern stations) were referred to at "Charlotte" and "Roger" originally. Anne Jermy (who is the most common voice of the Worldline system used at stations all over England) was referred to as "Sarah" originally. Eryl Jones (the Welsh announcer at TFW stations) was reffered to as "Glyn" originally. There are dozens of other examples. So i think people just do not know who they are and someone just makes up a name and then others copy and that becomes what they are known as.
Anita was found out to be Maggie Greenwood recently but she is still most commonly referred to as Anita as that is what everyone has known her as for so many years. She also used to do the English announcements on the Nuremburg U Bahn trains. I think someone said that Anita stood for "Annoying In Train Announcer" so presumably whoever came up with the name was not a fan of her announcements.
Someone else years ago said that Celia stood for "Creates Excellent Line Information Ambience" so there seems to be a few different acronyms that people have made up but of course Celia Drummond is her real name.
Vera apparently stood for "Very Erratic Repetitive Announcer" according to what i have heard from some people.
I have never been able to find out who Vera was but that is interesting that you mention Charlotte Green as that is the first time that i have ever heard any name.
Charlotte Green is the right sort of age and has done some voiceover work and worked at the BBC so was the sort of person who TFL would have chosen at the time:
en.m.wikipedia.org
You can listen to her showreel here:
Charlotte Green
excellenttalent.com
She does not really sound exactly like Vera but this was almost thirty years ago and voices do change. So it may be her. It is difficult to know for sure.
Unfortunately i have never been able to find any recordings of Vera on the C Stock trains.
Adrian Hieatt is his name. He is indeed a staff member. He is being used to replace Emma Clarke entirely on the refurbished 1992 Stock trains (and refurbished 1972 Stock trains too) which have started entering service. On the refurbished trains he announces the stations and destinations and everything. On the unrefurbished trains they have begun adding his voice for the manual generic announcements only.