Coalitions in the Commons?

To win a majority and sustain a government you need to be able to count.
A majority in the Commons is 321 supporting MPs, when allowing for the Speaker and Sinn Fein who do not vote.
The natural majority is therefore Conservative 317 plus DUP 10, giving 327.
The Conservatives could also form a majority with the SNP or Lib Dems on numbers, but there is clearly no wish on either side to do so, and huge differences of policy and attitude over Scottish independence, second referendum on the EU and other matters.
Labour would only form a majority coalition if it consolidated every party bar the Conservatives, which is also a political impossibility given the attitude of Labour to Northern Ireland and the DUP.

Some say as parties are split there could be a new coalition of the willing to put through a second referendum, or a Norway solution, or to cancel Brexit altogether. There simply are not the numbers to do that, all the time the two main parties oppose a second referendum and say we must leave.

We now know there are 71 Labour MPs willing to defy Mr Corbyn to speak out for a second referendum. With 35 SNP MPs and 12 Liberal democrats, also wanting a second vote, that makes a total of 108. In the unlikely event of Mrs May changing her mind over the desirability of a second vote, there would still be no majority for it, as at least the 110 Conservative anti Agreement MPs and the DUP would oppose it, a more numerous force than the 108 on the opposition benches. Mrs May does not want a second referendum. She presumably does not want to split the Conservative party on the issue. She must understand on current numbers a second referendum cannot pass. It is also very likely more than 110 Conservative MPs would defy any suggestion they voted for one.

We cannot be sure how many MPs want the so called Norway option. It is not official Labour policy and seems to have fewer Labour supporters than a second referendum, so similar considerations apply as with the second referendum. It has a couple of additional major problems. It would require the consent of the EU to a delay in Brexit, which would probably leave the EU saying the UK would still have to accept the Withdrawal Agreement and use the 21 month negotiating period in that to set it up if possible. It would also require the consent of existing EFTA members, and EU consent which would come if at all at a price. It would need the government to adopt that to carry out the negotiations.

There is then the official Labour policy, somewhat vaguely and erratically expressed, of staying in a customs union. This is sometimes linked with also being able to negotiate our own trade agreements, which would be incompatible with customs union membership. It is difficult to see how a majority would coalesce around this unless the government also made it official Conservative policy, which would detach more than 110 Conservative MPs from supporting the government and make much of the Department of International Trade redundant. It is unlikely the EU would ever consent to the UK being in the customs union, having its own different trade arrangements with others, and not having to observe the laws and freedoms of the single market and make financial contributions at the same time.