昨日紹介した国会図書館の概説３−２の引用だけでは、ここで関心のある天皇制に関する米本国の動きが不明なので、日本国憲法の基礎になっているSWNCC２２８骨子を見ておきます。
http://www.ndl.go.jp/constitution/shiryo/03/059/059tx.html#t005によれば以下の通り、ほぼ現憲法の（リフォーム）骨格が示されています。

Reform of the Japanese Governmental System（SWNCC228）
TOP SECRET
COPY NO. 66
7 January 1946
STATE-WAR-NAVY COORDINATING COMMITTEE
DECISION AMENDING SWNCC 228
REFORM OF THE JAPANESE GOVERNMENTAL SYSTEM
Note bytheSecretaries
１〜３省略（稲垣）
CONCLUSIONS
4. It is concluded that:
a. The Supreme Commander should indicate to the Japanese authorities that the Japanese governmental system should be reformed to accomplish the following general objectives:
(1) A government responsible to an electorate based upon wide representative suffrage;
(2) An executive branch of government deriving its authority from and responsible to the electorate or to a fully representative legislative body;
(3) A legislative body, fully representative of the electorate, with full power to reduce, increase or reject any items in the budget or to suggest new items;
(4) No budget shall become effective without the express approval of the legislative body;
(5) Guarantee of fundamental civil rights to Japanese subjects and to all persons within Japanese jurisdiction;
(6) The popular election or local appointment of as many of the prefectural officials as practicable;
(7) The drafting and adoption of constitutional amendments or of a constitution in a manner which will express the free will of the Japanese people.

ここで一応区切りますが、aによって上記７項目の（基本的人権の尊重や自由な選挙による国会の権能と政府・内閣が選挙民に責任を負う等）の基本原則（general objectives）が示されます。
命令／強制ではないというものの以下の一般条項が「should be 」・・されるべきであり、これを司令官は「should indicate」すべきということですから、この原則に反する憲法は認めない・・結局は、強制が目立たないように「巧妙にやれ」ということでしょう。
ですから、トップシーククレット文書でしたが、期間経過で公開されたので今では日本側の手に入っていることになります。
SWNCC２２８の続きです。

b. Though the ultimate form of government in Japan is to be established by the freely expressed will of the Japanese people, the retention of the Emperor institution in its present form is not considered consistent with the foregoing general objectives.

上記によればthe ultimate form of government（政体）は民意によって establishされるべきであるが、現状の天皇制を（そのまま？）維持するのであれば、上記７項目の基本ルールに適合するとは認められないと判定基準を示しています。

c. If the Japanese people decide that the Emperor Institution is not to be retained, constitutional safeguards against the institution will obviously not be required but the Supreme Commander should indicate to the Japanese that the constitution should be amended to conform to the objectives listed in a above and to include specific provisions:
(1) That any other bodies shall possess only a temporary veto power over legislative measures, including constitutional amendments approved by the representative legislative body, and that such body shall have sole authority over financial measures;
(2) That the Ministers of State or the members of a Cabinet should in all cases be civilians;
(3) That the legislative body may meet at will.

d. The Japanese should be encouraged to abolish the Emperor Institution or to reform it along more democratic lines. If the Japanese decide to retain the Institution of the Emperor, however, the Supreme Commander should also indicate to the Japanese authorities that the following safeguards in addition to those enumerated in a and c above would be necessary:
(1) That the Ministers of State, chosen with the advice and consent of the representative legislative body, shall form a Cabinet collectively responsible to the legislative body;
(2) That when a Cabinet loses the confidence of the representative legislative body, it must either resign or appeal to the electorate;
(3) The Emperor shall act in all important matters only on the advice of the Cabinet;
(4) The Emperor shall be deprived of all military authority such as that provided in Articles XI, XII, XIII, and XIV of Chapter I of the Constitution;
(5) The Cabinet shall advise and assist the Emperor;
(6) The entire income of the Imperial Household shall be tuned into the public treasury and the expenses of the Imperial Household shall be appropriated by the legislature in the annual budget.

以下省略

dでは、「The Japanese should be encouraged to abolish the Emperor Institution or to reform it along more democratic lines. If the Japanese decide to retain the Institution of the Emperor, 」として、日本国民が、天皇制廃止またはより民主的天皇制へのリフォームについてエンカレッジされるべきだが、民主的天皇制維持を決定したときには最高司令官は（これを尊重しながらも）日本当局者にCおよび以下の列挙事項（１）〜（６）を示すべきであるとし、そこには、現行憲法同様の内閣の助言承認や天皇の統帥権の剥奪、皇室年次予算の必要性などを書いています。
上記の通り１月７日には、既存天皇制を否定するだけではなく、むしろ存続させる方向性がありうることを示唆しています。
この時点では、戦犯として被告席に立たせる方向性が１００％否定されていたことが明らかです。