The closest parallel I think with recent politics now is with the John Major government in 1992. He had a majority that was weakened slowly through by-elections. It was economically incompetent. It ruined the railways. But for passing the Maastricht Treaty Major had a 'nuclear' option, a sort of Tory self-destruct. May does not have a nuclear button.
The first mistake is to rely on the Democratic Unionist Party. To hold Northern Ireland/ North of Ireland together needs the British government to be an honest broker without strategic interest in the province. To couple up with the DUP just in order to prop up a minority government is threatening a far bigger and delicate prize of long-term stability in the north. The DUP could have a shopping list in terms of the troubles memory and form of politics in the future that upsets the balance in Ireland.
The second mistake is to assume the government can have a 'stable' position regarding leaving the European Union. The problem for the EU side is that the negotiating partner in the British Government cannot deliver on its own deal.
Once again UKIP will be a very short tail wagging the Tory nutjob hard exit faction. So a deal that keeps the UK in the single market and customs union (that thus dissolves the border question in Ireland) will not satisfy these nutjobbers.
Yet a hard exit from the EU would not get past the vast majority of the House of Commons.
The clock is ticking and in two years the UK could fall out of the European Union. The Article 50 time limit and the cliff edge can only be prevented if all 27 in the EU agree - and the price may well be to stay in the European Union and no further 'time', no more Article 50, in order to prevent an economic disaster.
It is not clear either that a European Communities (Union?) Grand Repeal Act would get through and may be delayed until nearer an apparent deal.
The government, in other words, is heading for disaster, as it always was, but now doing so having lost political authority.
On anything else 318 or 319 is enough to run the House of Commons (the real majority line is 322 with the Irish abstentions) but always subject to Tory factions.
In the opposition Tim Farron should resign in favour of a more competent heavyweight and Jeremy Corbyn with authority but no power will be frustrated. It is quite possible after a time period to see a centre party realignment - the nearest to a grand coalition.
The Fixed Term Parliament Act should survive. The former prerogative is dead, and cannot be revived (otherwise monarchail prerogative is never removable and is superior to statute) so if the FTPA is repealed then some other democratic means to call a General Election is needed, not a Prime Minister who can dissolve a House of Commons as if the monarch did.