The question of approval or formulation of the democratisation of Japan is complex -do you mean those who formulated the overall policy of democratisation, or those who formulated the specifics of it?

Democratisation was one of the key platforms and aims throughout most US planning for the future of Japan from the earliest steps of planning from 1942. While there was a strand of debate that argued democratisation might not be absolutely necessary, some kind of change in Japanese government and administration was almost inherent in all of the various proposals for post-war Japan with the various governmental bodies in the US discussing the issue (largely State, War and the OSS). Democratisation included both negative and positive reforms and actions - both the removal of anti-democratic elements from Japanese government through political purges and the positive restructuring of government so that democracy would flower.

In formal terms, all of the key policy documents regarding Japan and its occupation stressed the importance of democratisation as one of the key objectives of the Occupation. The Potsdam Declaration of 26 July 1945 included such phrases as:
There must be eliminated for all time the authority and influence of those who have deceived and misled the people of Japan into embarking on world conquest, for we insist that a new order of peace, security and justice will be impossible until irresponsible militarism is driven from the world.
The Japanese Government shall remove all obstacles to the revival and strengthening of democratic tendencies among the Japanese people. Freedom of speech, of religion, and of thought, as well as respect for the fundamental human rights shall be established.
Both of these phrases express the desire for more free and democratic government in Japan.

The United States Initial Post-Surrender Policy (SWNCC 150/4/A) 6 September 1945 included as two of the ultimate objectives of the Occupation:
(b) Japan will be completely disarmed and demilitarized. The authority of the militarists and the influence of militarism will be totally eliminated from her political, economic, and social life. Institutions expressive of the spirit of militarism and agression will be vigorously suppressed. (c) The Japanese people shall be encouraged to develop a desire for individual liberties and respect for fundamental human rights, particularly the freedoms of religion, assembly, speech, and the press. They shall also be encouraged to form democratic and representative organizations.

The third major policy government JCS 1380/15 of 3 November 1945 included among the basic objectives of the Occupation;
the abolition of militarism and ultra-nationalism in all their forms; the disarmament and demilitarization of Japan, with continuing control over Japan's capacity to make war; the strengthening of democratic tendencies and processes in governmental, economic and social institutions; and the encouragement and support of liberal political tendencies in Japan. The United States desires that the Japanese Government conform as closely as may be to principles of democratic self-government, but it is not the responsibility of the occupation forces to impose on Japan any form of government not supported by the freely expressed will of the people.

Political democratisation
These three key policy documents formed the basis of the actions undertaken by SCAP (Supreme Commander Allied Powers - which refers to both MacArthur personally and is used as shorthand for the Occupation administration as a whole) from September 1945 towards the ends of democratisation. Some of these began almost immediately, and the first eight months of the Occupation involved the most intense period of political democratisation. Press and Radio Codes was issued by SCAP on 19 and 22 September 1945 starting the removal of restrictions on freedom of expression and information. On 4 October, SCAP issued SCAPIN-93 'Removal of Restrictions on Political, Civil and Religious Liberties which made a whole raft of changes - abolished the 1925 Peace Preservation Law used to control the population, all other ordinances and regulations restricting freedom of speech and expression, ordered the release of all political prisoners within 1 week, abolished all secret police organs, dismissed several ministers and officials, and removed lese-majeste laws. SCAPIN-93 also removed the restraints on political parties and allowed political parties to reform.

Another key initiative in democratisation was of course the new constitution which came into force in 1947. This was the result of American displeasure at the failure of the Japanese government to draft a constitution that met Allied objectives over late 1945 and early 1946. At the start of February 1946, the decision was made to draft a model constitution by SCAP to give the Japanese a better idea of what was expected. This was ordered on 4 February, and completed by the 10th. It was presented to the Japanese government on the 17th, and after internal debate, accepted as a starting point for negotiations with SCAP. With some minor modification and tweaks, the SCAP draft was essentially adopted as the official constitutional proposal of the Japanese government by 5 March. Under the existing Meiji Constitution which SCAP wished to operate under to help with the perception that the new constitution was a Japanese initiative, amendments had to be introduced by the Emperor and passed through the Privy Council and both houses of the Diet which involved further negotiations and some modifications. However, by 3 November 1947 this was completed and the Constitution came into force on 3 May 1947.

The new constitution of 1947 fundamentally reshaped Japanese government by turning on its head the existing structures. Whereas before, a quasi-absolute monarch and their political advisers in the executive, coupled with an independent military, dominated government, the new structure explicitly stated power was derived from the people, and that the supreme organ of government was the popularly-elected Diet. The constitution also greatly limited the powers of the monarch, and in great detail outlined the fundamental rights of the Japanese people.

Other aspects of political democratisation included the political purges and the International Military Tribunal for the Far East (IMTFE) which in part aimed at the removal of anti-democratic elements from Japanese government. The purges began in earnest on 4 January 1946 with two directives SCAPIN-548 aimed at organisations, and SCAPIN-550 aimed at individuals. These decrees largely involved specified categories which if any organisation or individual fitted into, they were abolished, or removed and excluded from government. Seven key categories existed - active exponents of militant nationalism or aggression, professional military officers, leaders of ultra-nationalist and terrorist societies, influential IRAA members (Imperial Rule Assistance Association - the attempt at creating a mass party organisation from 1940), officials of overseas financial and economic organisations involved in Japanese expansion, governors of former Japanese colonies and occupied territories, and 'additional militarists and ultra-nationalists'. This process was largely administrative in the sense that those purged were simply sacked or barred from office, rather than subject to any kind of trial or panel, and its aim was at least to provide a temporary 'clean slate' and allow more democratically-minded individuals to take office and bureaucratic positions.

Social democratisation
The idea of democratisation wasn't simply about making the government of Japan democratic, but making the people democratic as well. An important motiviation behind, and objective of reforms around education and information were on the basis of making the Japanese think and act democratically. The system of education existing in Japan as of 1945 was perceived to be antithetical to democracy as it was understood by the occupiers, and things such as textbook and curricula reform both to remove anti-democratic content. The existing system of rote-learning was to be replaced by teaching methods aimed to encourage students to form and express their own thoughts. Even things like reforming the administration of education towards more localised control was on the basis that American officials and educators (because of American historical practice and ideas) perceived centralised control of education as 'antidemocratic'.

I guess as a simple answer to your question - a whole range of people and groups formulated and implemented the democratisation of Japan which began very quickly after the Occupation began, but was an idea and objective in planning for an occupation from its earliest days in 1942.

This is a really hard question to answer in many ways, firstly because the operation that changed Japan into a democracy happened in in the 1800s and is commonly referred to as the Meiji restoration, and in fact the US occupation did very little to change the actual systems of government, it did however vastly and radically change the rules of governance. The 1946 constitution in many ways is actually an amendment to the Meiji Constitution rather than a clean break, while it does include some very drastic differences such as the renunciation of war, women's suffrage, the right to unionize, the reduction of the the Emperor to a head of state with almost no power whatsoever, among other items to modernize, progressive and liberalize the Japanese system of government. The fact of the matter is that the 1946 constitution was very much a collaboration between the Occupying Americans and the Remaining Japanese government, in fact the most famous article in the constitution, Article 9, was proposed by the then Prime Minister Kijiro Shidehara.

Honestly I think that someone with a better knowledge base can come in here and blow my answer out of the water, but the simple part of the fact was that the Americans never saw the Japanese government as needing to be fully replaced as they did the German government, in many cases the Americans never even truly followed through with many of their more restrictive ideas, and generally were gone by 1952(Okinawa and Iwo Jima, now Io Tou were given back in 1973 and 68). So to put it way way too simply the US basically passed the Japanese bill of rights through the Japanese Diet via normal political means, which while fundamentally altering the ways in which the government works did not repeal the Meiji Constitution.