I've seen the 2001 film 'Conspiracy' which dramatizes this meeting, and I have some questions regarding it:
I. Why does any record of this meeting exist? In film itself Eichmann mentions how secretive the meeting was, so why would they ever record it in the first place, and why weren't these records destroyed when the nazis were trying to cover up their crimes?
II. Why wasn't Heinrich Himmler at the meeting? I've often heard Himmler described as one of the architects of the Final Solution, and given his position at the head of the SS his presence seems like something that would be important when working out the logistics of murdering German's Jews.
III. Someone in the film says that prior to this meeting they had executed 30,000 jews, and suspect they still have another 20,000 to shoot. In this case, why was this meeting needed if Germans were already executing Jews in such high numbers?
The minutes of the meeting — the document generally referred to as the Wannsee Protocol — were intended as an aide memoire, to ensure that all participants clearly understood the new direction of Nazi racial policy, what was planned, and what was expected of them. Successfully carrying out a genocide on the scale of the Final Solution necessitated cooperation between many different wings of the vast Nazi bureaucracy; Heydrich's goal at Wannsee was to make explicitly clear the regime's policy towards the Jews, and to ensure that cooperation was forthcoming.
The complexity of the Final Solution as a logistical project, and the range of departments and offices it involved directly or indirectly, can be seen in the variety of officials who attended the meeting at Wannsee:
Peter Longerich, in an essay on Wannsee, contextualises the Protocol thus:
We do not know the precise wording of the statements made at the conference. Eichmann said in 1960 in Israel that he had to edit the minutes considerably at Heydrich’s insistence, and that the participants at the conference had used far more drastic language, and had spoken about deaths, elimination and annihilation. Eichmann possibly wanted thereby to divert attention from himself and incriminate third parties. In my opinion, the minutes should not therefore be read as a basis for speculation about what was 'actually' said at the conference, but as the guidelines authorized by Heydrich for the RSHA's allotted task of the 'final solution'. The starting-point for any interpretation of 'Jewish policy' at the beginning of 1942 should not be the actual proceedings of the conference, but rather their quintessence, which Heydrich presented to other supreme Reich authorities as the binding resolution of that meeting.
The meeting itself, and the distribution of the Protocol, were both conducted in utmost secrecy. Only 30 copies of the minutes were made — though we don't know exactly who was on that distribution list, beyond the 15 officials who attended the meeting. Only one copy, belonging to Foreign Ministry under-secretary Martin Luther, survived the war — most are presumed to have been deliberately destroyed in the final days of the Reich.
The Protocol is one of relatively few high-level documentary sources on the Final Solution that survived the Nazi regime's frantic attempts to conceal their crimes once the war turned against them. According to Christopher Browning:
Hitler operated in a very nonbureaucratic manner, verbally indicating his "wishes" and priorities. No paper trail leads to the Führerhauptquartier. At the next echelon, the files of Himmler and Heydrich regarding the Final Solution were destroyed. The historian is left with copies of a few key papers — such as the Göring authorization, the Einsatzgruppen reports, and the Wannsee protocol — that Himmler and Heydrich sent to others, but not with the vital internal working papers at the coordinating center.
To your second question: Himmler's absence from the meeting isn't particularly unusual: as head of the Reichssicherheitshauptamt, Heydrich oversaw the entire SS security, intelligence and state terror apparatus: the SD, Gestapo, criminal police and the Einsatzgruppen paramilitary death squads were all under his control, which made him the logical overseer for the genocide. Heydrich was also among Himmler's most trusted lieutenants, so it's not especially surprising that the tasks of corralling bureaucrats and organising the practicalities of the Final Solution were delegated to him.
To your third question: there's a significant difference in logistical complexity between mass executions (even in the order of tens of thousands), and the much larger project of annihilating a European Jewish population that the Final Solution's architects estimated at over 11 million people — especially given that the Nazi regime intended that the Final Solution be conducted as secretly as possible.
By the time of Wannsee, SS officials had concluded that the Einsatzgruppen approach was inefficient, and that the use of firing squads to exterminate populations en masse had a deleterious effect on the shooters' morale. By mid-1941, the SS were already experimenting with gas vans as an alternative, more 'hands-off' method of murder; in October 1941, construction began on the first dedicated extermination camps, equipped with gas chambers and crematoria. The purpose of the Wannsee Conference was to organise the deportation of Europe's Jews to these six extermination sites in occupied Poland (Auschwitz, Bełżec, Chełmno, Majdanek, Sobibór and Treblinka) — that's what the euphemistic phrase "evacuation to the East" in the Protocol means.
Some recommended reading on Wannsee and the architects of the Final Solution:
Longerich also has a new book on Wannsee due to be published later this year — he's one of the absolute top tier of Holocaust historians, so it should be an interesting addition to the historiography.