The Scottish economy may be going downhill fast, but there was no shortage, yesterday, of post-Brexit irony to keep us amused. There was the Tory Scottish Secretary, David Mundell, meeting with business figures in an effort to reassure them that independence (from Europe) will not damage the economy, despite all the evidence to the contrary. Meanwhile, Nicola Sturgeon was returning from Germany where she'd appealed for help to remain part of the Union (the European one). It's like a parody of what might have happened had Scotland voted Yes to independence in 2014: oil crisis, financial uncertainty, relations with Europe. Except for one thing: the Scottish Government doesn't have the economic powers of independence to meet the demands of the time.

This was underlined yesterday by Ms Sturgeon’s £100 million economic stimulus, which is scarcely worth the name – especially since it isn’t exactly new money but unspent cash lying around from last year. Of course, a boost to NHS treatment centres, bringing forward infrastructure investment and a counselling service for anxious businesspersons is a Good Thing. It certainly won rave reviews from the Scottish Chambers of Commerce boss Liz Cameron, who seems to be doing the Scottish Government's PR for them. But as Labour's Jackie Baillie said, £100m is a “drop in the ocean”.

So what else could the First Minister do to head off the recession that seems to be thundering down the road towards Scotland? Well, her government could slash income taxes across the board as some Conservatives advocated before the Holyrood elections. That might put more money in people's pockets, but the resulting cuts in public services would not appeal to Scottish voters. She could press ahead with fracking to replace the depleted North Sea Oil industry, but there isn't a majority in parliament for that, whatever her own views on the safety or otherwise of unconventional gas extraction. She seems determined to press ahead with cuts to Air Passenger Duty even though Labour say it will only benefit the better off and will have an environmental cost.

Of course, it's not Ms Sturgeon's fault she is fighting a tank war with a pea-shooter: the Scottish Parliament is not designed to deal with an economic crisis of this scale. It has very limited borrowing powers, no control of business taxes and has no independent representation in the EU. The Scottish Government doesn’t have the authority to launch mass house building programmes, embark on comprehensive road renewal, or finance the renewable energy projects that might make a real difference. Even in areas where Holyrood nominally has control, like energy policy, its real power is limited.

Take North Sea decommissioning. The sight of the Transocean Winner oil rig bumping up against the Lewis coast is a reminder the big money in the North Sea in the coming years is going to be in the area of dismantling rigs, sealing wells and lifting the pipelines that took Scotland's oil wealth to the mainland. It is a poignant moment – not least for the SNP who have always seen oil as being the guarantor of Scotland's economic independence. But there could still be rich pickings from the hydrocarbon carcass in the North Sea. Unfortunately, it looks unlikely they will be Scottish or even UK companies which will get to do the picking.

Norwegian firms have snapped up the contract for the Maersk field because – wait for it – Scotland doesn't have the facilities or track record to decommission its own oil fields. This will be seen by many Nationalists as the final insult in the history of Scotland's oil. Not only was Scotland the only country, state or region in the world to discover oil and not directly benefit – even in Alaska citizens get an annual dividend – it isn't even going to benefit from the work needed to clear up the mess.

Well, Scotland voted No to independence and so there is little point wondering what might have been if Scotland had left the UK. It would anyway have been difficult for an independent Scotland to cope with the current global crisis in the oil and gas industry. But the SNP insists, legitimately, that an independent Scotland would at least have had the full range of powers to manage the economy in Scotland's interests, most notably by remaining in the EU.

The Scottish Government's entire economic prospectus for independence rested on being part of the EU. It was the guarantor of economic continuity. EU membership ensured an independent Scotland would not be a tiny country isolated from the trading relations of the world. It's now Scotland's luck to find itself so isolated while lacking the economic powers that would come with independence.

As part of the European Single Market, Scotch whisky exports – worth £4 billion annually – were exempt from tariffs, not just in Europe but across all the countries that had negotiated trade arrangements with the EU. Now whisky exports to Korea could find 20 per cent import duties slapped on our national drink according to the Scotch Whisky Association. Scotland will have no autonomous role in the negotiations over post-Brexit trade deals.

But to give the SNP leader credit, she has not just folded her arms and waited for the coming recession to hit Scotland so she can say it's all Westminster's fault. The First Minister is trying to elbow her way into trade negotiations. The Scottish Government wants to show it is doing something on its own account to head off the Brexit downturn, even though there is very little it realistically can do.

The inconvenient truth is that, as far as Brexit is concerned, we really are all in this together so long as Scotland remains part of the UK. It is very difficult to see what the Scottish Government alone could do to prevent the recession economic forecasters are predicting. The Institute for Fiscal Studies calculates Scotland and the UK stand to lose 4 per cent of GDP as a result of not being in the single market. That's a major recession and over and above the one the UK is already facing post-Brexit.

Perhaps the most important action SNP MPs in Westminster could take is to throw their weight behind Labour leader Jeremy Corbyn's demands for a serious UK-wide economic stimulus of £500bn. We are in the extraordinary situation where even Tory leadership candidates, like Stephen Crabb, have been talking about a stimulus worth hundreds of billions. The SNP could also unite with Labour in an effort to keep the UK in the single market – for which there is almost certainly a majority in both houses of parliament.

Brexit may mean Brexit, but the UK Government is divided within itself over what Brexit means. Prime Minister Theresa May wants to remain in the European Economic Area and therefore within the single market. But this will involve payments to the EU and free movement, which is why many Tory MPs wanted to leave the EU in the first place. But they are a minority. This presents an opportunity for the opposition in Westminster where the SNP is the third largest party and has a strong moral voice. The game isn't over; it's hardly begun.