Were there concrete plans, 40-41, for Japan to attack Singapore/Indonesia, while avoiding a fight with the US?

Avoiding conflict with the United States was apparently briefly considered and rejected.

Found in the postwar Japanese monographs, number 150: Political Strategy Prior To Outbreak Of War Part IV starting on page 7:

Operational Policy Against the United States, Great Britain and the Netherlands East Indies

With the complete breakdown of negotiations between Japan and the Netherlands East Indies on 18 June 1941, Japan became highly apprehensive of her vital oil supply. It was feared, also, that unless Japan took the initiative, that the United States and Great Britain, by launching a political, economic, and military offensive against Thailand and French Indo-China, would gain control of those countries. In order to cope with this situation, therefore, an Outline of the Empire's National Policy to Cope with the Changing World Situation was decided upon at an Imperial Conference on 2 July, and, in accordance with this policy, Japanese forces entered southern French Indo-China on 31 July. (Monograph 147, Political Strategy Prior to the Outbreak of War, Part III). This move was made in order to strengthen Japan's strategical position and to enable her to threaten force against the Netherlands East Indies, to compel that country to come to terms in regard to oil. To the complete surprise of both Imperial General Headquarters and the Japanese Government, this led to a general United States embargo against Japan. Japan had hoped to adjust the situation by diplomatic negotiations but the United States and Great Britain applied drastic economic pressure by freezing Japanese assets overseas and placing an embargo on oil to Japan. This, together with the Netherlands East Indies' refusal to supply Japan with oil, virtually forced Japan to study plans for war against the United States, Great Britain, and the Netherlands East Indies.

An operational policy was formed by supplementing, unifying and adjusting the operational policy against the Netherlands to the annual operational policy against the United States and Great Britain. In forming this policy it was necessary to attach primary importance to the various requirements of a war plan and to subordinate the operations policy to them due to the paramount importance of Japan's seizing the southern area, rich in natural resources in the shortest possible time, before her domestic resources were depleted. (The term operational policy as used here refers to military plans for the armed forces. War plan designates mobilization of the entire country for total war, to include manpower, industry, transportation, etc.)

On 29 July 1941, having in mind President Roosevelt's announcement of 24 July that the United States would impose an embargo on oil shipments to Japan unless the Japanese forces were withdrawn from French Indo-China, President of the Planning Board Suzuki felt it necessary to submit to the Government and to Imperial General Headquarters a report of the resources it would be necessary for Japan to mobilize in the event of war, and urged them to execute a war aimed at the acquisition of resources in line with these demands (Appendix 1). He concluded the report by saying:

Since the actual condition of Japan's material resources is as stated in this report, once we have formed actual plans for war we must adhere to them and not be swayed by local changes in the international political situation. Under present conditions, it is extremely difficult to develop our national power by depending on the United States and Great Britain for the acquisition of materials. If steps are not taken to remedy the position, Japan will find herself completely without resources and unable to defend herself in the future. Japan, therefore, is now forced to make a final decision without hesitation. In view of the inseparable relationship between the southern and northern areas of the international political situation, it is hoped that a command plan will be formed to guide armed warfare so as to convert operational gains into productive use in the shortest possible time.

Under such circumstances, even the minimum demands of a materials mobilization plan could not be met unless Japan seized the southern area, rich in natural resources, at the outset of war and gained command of the sea and air by swiftly destroying the military strength of the United States, Great Britain and the Netherlands in that area. With this in mind, the following operational plans were carefully studied.

1. An attack to be launched upon Malaya and the Philippines, after capturing the Netherlands East Indies during the opening phase of hostilities.

2. Advance to be made clockwise along the operations line running from the Philippines to Borneo, Java, Sumatra and Malaya.

3. Advance to be made counter-clockwise along the operations line running from Malaya to Sumatra, Borneo, Java and the Philippines so as to delay the outbreak of war with the United States as long as possible.

4. Attacks to be made on the Philippines and Malaya simultaneously and then successive and swift advances to be made southward along these two lines of operations.

Several discussions took place between the Army and Navy High Commands in regard to these four plans. It was agreed that Plan No 1, whereby the Netherlands East Indies would be occupied in one swift movement, leaving at the rear of the line of operations, the United States and British powerful bases of the Philippines and Malaya was not feasible. The Navy was in favor of Plan No 2 while the Army insisted on Plan No 3. The Navy felt that Plan No 2 was the best method from the standpoint of sequence for the concentration and employment of forces and extension of the operations line. It was feared, however, that by the time Sumatra and Malaya were attacked, that their defenses would be so powerful that they could be able to withstand our offensive. Plan No 3 might, as a political maneuver, delay the United States' participation in the war, as Germany asserted it would, by by-passing the Philippines. Even if Malaya and the Netherlands East Indies were occupied as strategic moves, however, if the powerful United States Navy and Air Forces were deployed to strengthen the Philippines, they could possibly disrupt our line of operations and cause us to abandon already occupied areas. Moreover, if the attack on the Philippines, the most powerful area in the line of operations, mere postponed, eventually these islands would become impregnable.

After serious study of the merits and demerits of the plans, Plan No 4 was finally adopted. This plan called for the swift advance southward of our forces along the two lines of operations to the oil fields in simultaneous operations in the Philippines and Malaya, insofar as strength permitted.

This was the situation about the middle of August 1941.

The Japanese Monographs were produced by the Military History Section Headquarters, Army Forces Far East through the collaboration of Japanese participants. The link above is a transcription. In their original production they can be found here