Who is Lucius Malfoy's informant?

[pigwidgeon37 at yahoo.it]

No: HPFGUIDX 21396

I'm re-reading CoS at the time and something is bothering me:
Dobby came to Harry in Privet Drive to warn him about the danger he 
would be in when returning to Hogwarts. His exact words are: "There 
is a plot, Harry Potter. A plot to make most terrible things happen 
at Hogwarts...this year.<>Dobby has known it for months, sir...."

1) How did Dobby come to know about the plot? Certainly overhearing 
Lucius talking to somebody about his evil intentions. But who did he 
talk to? Now, IMHO, not to his wife Narcissa. This is just a feeling 
of mine, but based also on the fact that Narcissa isn't a DE (we 
know, from the Lestranges, that there are DE couples). I don't think 
Lucius talks "politics" with her. So the big question is: Who did he 
talk to?
2) In Cos, the Weasleys go for the traditional Diagon Alley-shopping 
on some day in August, not on the last day before school starts, as 
they do in PoA and GoF- just some day. The question that seems 
obvious to me is: How did Lucius know they were to go there on this 
particular day? The apriori for putting the problem this way is of 
course that he was targeting Ginny in the first place, which I 
believe he did: She was in many ways the ideal victim- girls of that 
age are more likely to write diaries than boys are; and certainly the 
youngest sister of six brothers would have something to write! Of 
course, he could've put the diary in some of the school books just at 
random, somebody would have bought it anyway, but it might have been 
a 7th year boy.
Riddle has certainly told Lucius WHAT to do, but who told him WHEN to 
do it? 

3) I'm not an English native speaker, but unless I'm very much 
mistaken, a plot is something that requires more than one person?! 
Excluding Narcissa, as I did above, this seems another clue to 
somebody else being involved.

Maybe someone's got suggestions?

susanna