The Muslim Leage, demanding Pakistan, got 90 percent of the Muslim vote in January 1946. The Brits hoped that India would remain united but the League wanted partition. 

 The Cabinet Mission Report of May 16 1946 takes note of this- 

4. It is not intended in this statement to review the voluminous evidence which has been submitted to the Mission; but it is right that we should state that it has shown an almost universal desire, outside the supporters of the Muslim League for the unity of India. 5. This consideration did not, however, deter us from examining closely and impartially the possibility of a partition of India; since we were greatly impressed by the very genuine and acute anxiety of the Muslims lest they should find themselves subjected to a perpetual Hindu majority rule.

Hardly a problem if you are the majority- as would be the case in Muslim majority provinces. Thus the indigenous people of what would become Pakistan had been living under Muslim rule, not Hindu rule, since 1937.  

This feeling has become so strong and widespread amongst the Muslims that it cannot be allayed by mere paper safeguards.

Muslims in Hindu or Sikh majority areas would be subjected to ethnic cleansing so as to accommodate refugees streaming across the new border. 'Paper safeguards' were known to be worthless from about 1934 when Poland unilaterally repudiated Minority Treaties.  

The safeguard the Mission should have looked at had to do with an orderly exchange of population supervised by the Armed forces and facillitated by elected politicians and government employees. 

This is because the Muslim League had failed to reach an agreement with Congress.

 we… the Cabinet Ministers and the Viceroy… have done our utmost to assist the two main political parties to reach agreement upon the fundamental issue of the unity or division of India. After prolonged discussions in New Delhi we succeeded in bringing the Congress and the Muslim League together in conference at Simla. There was a full exchange of views and both parties were prepared to make considerable concessions in order to try to reach a settlement, but it ultimately proved impossible to close the remainder of the gap between the parties and so no agreement could be concluded. Since no agreement has been reached, we feel that it is our duty to put forward what we consider are the best arrangements possible to ensure a speedy setting up of the new constitution.

Guys who knew nothing about India were saying they had the solution for its political problems. This was bound to fail.  

This statement is made with the full approval of His Majesty's Government in the United Kingdom.

Which was bankrupt. Atlee wanted out of India as soon as possible. His Viceroy had a plan to evacuate the White population.  

If there is to be internal peace in India it must be secured by measures which will assure to the Muslims a control in all matters vital to their culture, religion, and economic or other interests.

Sadly, Muslims are quite capable of slaughtering each other. Also, if Muslim kill kaffirs, kaffirs retaliate where they are stronger.  

We therefore examined in the first instance the question of a separate and fully independent sovereign state of Pakistan as claimed by the Muslim League. 

This was a waste of time. The army and civils service was already polarised. The problem for Jinnah was that Punjab might revert to a 'Unionist' coalition while Bengal might come under a KPP coalition. The League's moment of glory would be evanescent. That is why he was in a hurry. 

The Cabinet Mission rejected 'greater Pakistan' on the basis that non-Muslim majority districts would not accept it. The League, at that time, would not accept the smaller Pakistan which they eventually got. This is the reason the Mission rejected partition. 

We are therefore unable to advise the British Government that the power which at present resides in British hands should be handed over to two entirely separate sovereign states.  This decision does not, however, blind us to the very real Muslim apprehensions that their culture and political and social life might become submerged in a purely unitary India,

If it hadn't been submerged during Maratha or Sikh or British rule, how would it become so if Congress- which contained a fair number of Muslims- formed a governing coalition at the Centre.  

in which the Hindus with their greatly superior numbers must be a dominating element. To meet this the Congress have put forward a scheme under which provinces would have full autonomy subject only to a minimum of central subjects, such as foreign affairs, defence and communications.

This was what the 1935 Act had envisaged- a Federal India not too dissimilar to what already obtained. Why did the Mission reject it? The answer is that it was using the chimera of 'Grouping' to satisfy the Pakistan demand.  

Under this scheme provinces, if they wished to take part in economic and administrative planning on a large scale, could cede to the centre optional subjects in addition to the compulsory ones mentioned above. 13. Such a scheme would, in our view, present considerable constitutional disadvantages and anomalies. It would be very difficult to work a central executive and legislature in which some ministers, who dealt with compulsory subjects, were responsible to the whole of India while other ministers, who dealt with optional subjects, would be responsible only to those provinces who had elected to act together in respect of such subjects. This difficulty would be accentuated in the central legislature, where it would be necessary to exclude certain members from speaking and voting when subjects with which their provinces were not concerned were under discussion. Apart from the difficulty of working such a scheme, we do not consider that it would be fair to deny to other provinces, which did not desire to take the optional subjects at the centre, the right to form themselves into a group for a similar purpose. This would indeed be more than the exercise of their autonomous powers in a particular way. 

This is specious reasoning but the purpose is obvious. Jinnah & Co. could claim that 'grouping' was almost the same thing as 'greater Pakistan'. But, it had to be backed up by the offer of seccession after a ten year stand-still agreement.

The Cabinet Mission offered a very weak Centre with Provinces and Princely States retaining all residuary powers. The kicker was

5) Provinces should be free to form groups with executives and legislatures, and each group could determine the provincial subjects to be taken in common.

Could the whole of Punjab and Bengal/Assam compe into such groupings?  If so, greater Pakistan would exist in all but name. 

(6) The constitutions of the Union and of the groups should contain a provision whereby any province could by a majority vote of its legislative assembly call for a reconsideration of the terms of the constitution after an initial period of ten years and at ten-yearly intervals thereafter.

Did this mean secession? Yes- if big enough 'groupings' were involved.  The problem was that nobody could be sure how people would vote in five years time- let alone after a decade. The Muslim League had a narrow window of opportunity. 

One final point. The Brits owed a lot to the Sikhs. They were going to leave them lurch. Thus, they pretended otherwise- 

We think that for these purposes it is sufficient to recognize only three main communities in India, General, Moslem and Sikh, the "General" community including all persons who are not Moslems or Sikhs.

Sadly, the Sikhs found that, when it came ethnic cleansing, they clustered with the 'General' community.