One thing I forgot to add, the biggest risk of disruption after Brexit is going to be from jobsworths who will query every single process that they carry out and will not do anything with express instructions from their management, regardless of the consequences.

Think of traffic wardens who will ticket a car who's driver fas just dropped dead of a heart attack.

The main difference I see between the GFC and Brexit is that the GFC was everyone in the West's problem. They all had a vested interest in sorting it out. Is it in the EU's/US/China's stategic interset to dig Britain out of a hole?
The GFC only affected one part of the economy, the banks. Brexit is all parts of the economy, it also affects defence, education, transport, justice. That's a lot of late nights, they'd want to be getting started last year to hit March '19.
Brexit is a regional problem, mostly Britain and Irelands with it's ramifications lessening as it radiates out. I can't see the US power structures sitting through the night like it did for the GFC to pass rules to allow Britain to trade as it does presently under the EU. Will the Chinese politburo?

If they want to avoid chaos, they will.

But who is "they"? If this was just the UK and EU in the mix, I'd agree with you.
And WTO MFN status hangs over all of this. If the EU or UK decides to break international law by allowing goods to continue to circulate after March 29 (without a transition period or FTA), as they do now, all other nations in the WTO will land on them like a ton of bricks.

The options to avoid chaos are massively constrained by the change of status of the UK into a Third Country, and the expectations of all other WTO nations as to how they will be now be treated by the UK concerning UK imports. Tariffs are one thing, but I understand (but may be incorrect) that MFN also applies to sanitary and phytosanitary measures. So both sides have to put in place measures that don't expose them to WTO litigation by any other nation. Outside of a transition period or FTA, the EU has to then strictly treat the UK as a Third Country.
That is, if Trump doesn't decide to try destroy the WTO first. This isn't unthinkable anymore. God knows what would happen then.

Given the UK does a lot of trade with the rest of the world under EU trade agreements, what will happen there, we are all aware of how the EU will treat them but by the sounds of it the ROTW will treat them the same if there is a risk of WTO litigation on anything other than WTO rules without separate trade agreements? I've read nothing of trade negotiations with the ROTW.

In the time left, I think only an EEA/EFTA agreement is now possible for the UK. In fact there is some debate as to whether or not this is the default status of the UK after March 29th. The UK did not trigger EEA Article 127 last year; they say that A50 automatically covers it, but international law experts disagree. The UK could reverse itself at the last moment and declare they are members of the EEA after all.

One wonders if this is the reason for the sangfroid the EU is showing.

May would have to stand down by then. Boris would then be the most likely candidate to succeed. With a Euroskeptic PM in charge, EEA/EFTA could then be presented as a stepping stone along the way to a magical bespoke agreement in the far future. It could be sold as a face saving measure - the EU is backing the UK into a corner, so the UK (really the Tories) are going to play for time.

ERG may go for it on that basis as long as Boris (or some other noted Euroskeptic) is in charge. That this is a stepping stone, not the end of the road for the process.
Remainers may go for it assuming, by default and by stint of time, that the situation bogs down and stabilizes.

This assumes of course the ERG don't believe their own ridiculous rhetoric of the benign no-deal scenario.

Scarily, I think they actually do.

I expect that global traders will continue to operate under existing rules until new rules supersede them, after all in the real world they want minimal disruption.
To take a silly analogy, you're driving down a road and come to a section where the white line has worn away or is missing due to road works, would you stop driving because you can't see the centreline any more or continue to drive but keep to the left, many here seem to believe that traders will do the former.

Good analogy but just like driving along the Borderlands it's easy to get caught out by signage for the speed limit changing between klm and mpr.

It is in everyone's collective interest to pretend that nothing has changed, but it will not be in individual's interest to do so.

Sometime after the Brexit date there will be a mundane routine issue. There will be a car accident, or something similar, which will leave a number of people seriously injured and the insurance company will be on the hook for millions in claims. It may be callous to call this a routine issue, but accidents like this happen every day. When this happens the insurance company will look through all the legal agreements in minute detail and come up with a Brexit related reason why they don't have to pay. A car accident results in millions in insurance claims, airline accidents results in hundreds of millions. The numbers are just too big to pretend and play along that nothing has changed.

Does anyone really believe the insurance companies will pay out millions when they don't have to?

I'm not sure it is a good analogy. How about this one: you're flying on holiday and they don't check your passport on the way out. When you arrive in Britain, your passport is expired. What do you reckon the British border force do with you? My guess is they ship you right back on the next flight from whence you came. The uncertainty at the point of send means goods won't be accepted for send. It may be that some goods are already in flight and will be accepted under the 'old' rules, but this is a delay.

Back to your road analogy. Imagine if the rule that said you drove on the left also was no longer in force and it wasn't clear what replaced it. The roads become a freeforall, would you be happy to drive on them?

edit: and, as the Curious One says, you're also no longer insured... or might not be...

So when it goes bust, we bail it out. What can go wrong

It'll be the ECB on the hook next time will it not?

Don't think so, so long as the ultimate parent remains City of London regulated entity, it is up to the UK to step in to protect it's subsidiary