The eavesdropper: DD plant

[confusinglyso]

No: HPFGUIDX 87928

I wonder whether there really was an eavesdropper.
Trelawney is being interviewed by Dumbledore when
she goes into a trance to produce the prophecy.
1) Dumbledore must give her full attention so that he does
   not miss any detail, (or could 'pensieve recall' overcome this
   problem?)
2) Fracas to eject "eavesdropper" could cause trance to be broken.

DD believes the prophecy but has to let LV know part of it to set 
events in motion.

The Lexicon shows the Timeline as follows:
DD interviews Trelawney in Spring 1980 (possibly fall 1979)
Harry is born 31 July 1980
DD hires Snape 1980, presumably for September term start.

Snape is reformed DE. DD trains Snape in Occlumency/Legilimency.
DD then sends Snape to LV with the tale that he had overheard the 
beginning of a prophecy concerning LV and one who would vanquish him.

DD seems to be totally driven by the prophecy as other posters have
pointed out.

My only problem with my theory is OoP Chapter 'Educational Decree #24'
page 323 UK edition, where Umbridge is questioning Snape on how long
he has taught at Hogwarts and he replies 'Fourteen years'.
This reply contradicts the Lexicon, can 15 years be supported from
other canon. Harry is 15+ in OoP.
Could Snape be lying to Umbridge about his length of service?
Why would he need to lie about this?

Phil