Well not being able to quanify a benefit of being better off outside the EU is exactly why I think its a bad idea to leave. That's not a criticism of your argument/position; thats a criticism of the whole lack of debate and lack of evidence used when arguments are made for Brexit.
There are many ways to show how Britain will be worse off - from losing out on trade, losing a lot of businesses to other EU countries (many companies and organisations have already left), massive capital flight, much more convoluted trade arrangements, increased costs of transport for trading further afield (not to mention the carbon footprint impact of this), the negative consequences of the trade deals that will have to be taken with no real bargaining power - the list goes on and on. The most viable option appears to revolve around becoming a deregulated tax haven which will likely not be good news for the vast majority of the population - nor will it be popular with Britain's neighbours - and certainly won't help with many trade deals.
While a lot depends on the deal - one of the biggest sticking points (which we haven't covered yet) is the Northern Irish border and the Good Friday Agreement - but I digress here.
The only options so far are: May's deal - dead in the water, rejected by parliament (and rejected by the hardest Brexiteers) - this deal would leave Britain taking rules from the EU but without having a say in the creation of these rules. Britain also has a veto currently - so it is in a pretty good position as a member but will be giving up a lot and would gain very little from this arrangement, by giving away influence for no real benefit.
There is no deal - but this would be disasterous for all kinds of reasons. Firstly, WTO terms don't cover Britain's biggest export - services. But secondly - there would be no regulatory allignment on anything - the list of things that would be affected but aren't being discussed could fill a hefty book so we can revisit this sub-point at a later date. No deal would hit the country hard and devalue the pound considerably - so all these allegedly cheaper imports wont be so cheap with the pound taking a dive. Look at the strength of the pound versus pretty much any currency you like through this summer - 30 year lows against the dollar for starters! Again, the majority of people in Britain will be hit by this and those who will be affected the most are those least able to afford it.
There is Borris' deal - which nobody knows anything about. Borris says he's making progress with this - the EU says he's tabled nothing of what his plan actually is. Borris isn't known for honesty having lost a job in journalism for making up quotes - his former boss says he's utterly unfit to be prime minister. So let's move on from that waste of time.
The only other option currently available is to revoke article 50 and cancel this utter mess completely.
Back to your point about worse off in the short run - yes, Britain will be a lot worse off in the short run - but its unlikely to get better for a generation. So I would argue the medium term and probably the long term would also be worse off outside the EU.
Yes - if exiting the EU Britain would need more trade deals, but therein lies the problem. The Brexiteer's argument is that we could go zero tarrif - but that would have to be applied to all countries. Once they have that, then whats left for them to ask for in a trade deal? They would already have the biggest benefit of zero tarrif access to the market without even having to reciprocate! Not to mention that most trade deals take many years to negotiate, or the fact that gaining access to the world's largest trade block (the EU) is very desirable, but gaining access to Britain as an individual country would be far less important to a trading partner.

On to your point about business adjusting to new markets - yes, they will need to adjust. Membership of the single market is a trade enabler - not a trade inhibitor. It standardises regulations and massively simplifies international business. Stepping away from this trading block will greatly increase complexity and cost - damaging the economy and causing previously successful businesses to fail and many more to struggle.

The conduct of the conservative party has been appauling - I would argue less so from parliament. I would also disagree with your point about only having to worry about one appauling government if Britain left the EU - the risk of becoming insignificant is significant. The fact that Borris prorogued parliament shows that Britain is becoming more authoritarian and the people will have a great deal to worry about if that continues. The EU is far more democratic than is implied in the right--leaning British press.

It would appear that most people in the UK think that in another referendum that the result would be to remain. That's why so many Brexiteers are so opposed to one. How can an action of direct democracy be argued to be undemocratic? Especially by the people who claim to be enacting "the will of the people". Again, the hypocracy of some of the Brexiteers here is appauling. Reese-Mogg advocated for multiple referendums before the surprise win for leave - now he fights against what he previously advocated for. Farage said that the argument would be far from over if remain won by a small margin - so why should it be far from over when leave won by a small margin?
And let's not forget that people are far more clear about what leaving actually means now. Even if we ignore the underhand practices that went on during the 2016 referendum, people being more informed and voting on the matter again would break the deadlock whichever way it went. How anyone can claim to be representing the will of the people while denying them a say in anything and shutting down all debate is parliament is an affront to democracy in my view.