Can derivatives trading, stock markets or fiat exist in a global Credit Commons?

The mutual credit is an accounting system that emphasises balance i.e. giving and receiving in equal quantity. The notion of credit limits is intended to prevent accumulation of credit and debt. The builtin sufficiency of credit is designed to prevent rich traders from profiting merely from the fact of their trade surplus.
However the financial instruments you describe exist on a different level to the accounting system. It would be possible to build complex financial instruments on a mutual credit system but some rules would need to be in place
- implement large credit-debit limits
- restrict the access of some or most members to credit, or make it costly.

Financial systems emerge from value systems. In doing mutual credit we are showing that the social technology of the medium of exchange is not tied to notions of commodity money and all the evils thereof.
The credit commons as presented in the white paper is more suited to debt instruments than asset instruments. Debt is always denominated in social agreed units of account. Asset instruments change in value and are traded on markets very different to the system of negotiated exchange rates described in the credit commons.

I think the vision we are creating is of a social transformation that grows within the existing economy, and comes to be seen as 'the new common sense', a way of living that really works for people and planet. It not only makes sense, but also appeals to people's hearts and sense of spirituality. Thus I think that if it can get started, (and I don't underestimate either the difficulties, or the potential for serious opposition and disruption) it will soon reach a take-off point at which it grows really quickly. That is what I would like us to be aiming for.
In that vision, it doesn't make sense to have resources put into a project because of the prospect of financial return rather than caring for people and planet. Stockmarkets, derivatives, venture capital are core parts of the social madness, as are wars and the armaments trade, and disruption of ecosystems for profit. So the issue becomes, for larger projects, how can we get agreement for and organise the commitment of large amounts of resources? There are lots of ways in which this can be done. We shouldn't be trying to find a niche for stock markets, derivatives, etc. They are the problem!

Agreed. I'm putting in lots of my time voluntarily on this, not to help small businesses with their cashflow (although that is a happy by-product), but to change the world - and by that I mean to help build a new kind of economy, based on co-ops, commons and sole traders, with no place for stock markets, currency speculation, money lent at interest or landlordism.

I'm not convinced we can do it before nature sweeps us away, but I don't see anything better that we could be doing instead.

anyone convinced that we can do it in time is surely deluded, given the state of nature and all that, and I expect we all agree that such an assessment requires we do the best we can in the circumstances.
So the question is - as always - what's the best we can do?

Doing it in time? The best we can do? If we can get a foothold, the change will happen very much more rapidly than most people can imagine, so it isn't clear how much we will be able to rescue. And the network of networks of mutual support IS the best we can do. It creates the resilience at all scales to handle whatever is needed.

"The creatures outside looked from pig to man, and from man to pig, and from pig to man again; but already it was impossible to say which was which." That said, I thought about this, a few years ago. I ended up in favour of simple derivatives, like harvest futures, but 'thus far and no further'. Apart from that, I'm interested in what might happen via stigmergy: https://en.wikipedia.org/wiki/Stigmergy if a bunch of mutual credit nodes, coops etc. began to interact. That might provide a basis for building something a little less destructive.

Hmmm, the word stigmergy is new to me, but the concept is, of course, familiar. I recently put together this 30 second video meme that demonstrates it, and shared it with a few of our group. Have a look:
https://youtu.be/wap-Y3mENVw.
I'm trying to imagine how humans might emulate the ants.

My understanding of why people like stigmergy is that a small number simple principles are sufficient to generate rich compex behaviours and structures, without requiring detailed design and planning. The prime example is the complex and sophisticated termite mound with its ventilation and climate control, but without any architects guiding it. For our emerging human society, it means working together with goals the wellbeing of people and planet, at various scales. No grand five year plans needed!

But there must be some program somewhere that causes the termites to do what they do even if the individuals are not "aware" of it. Is that what is called "instinct?"

Maybe humans have "instinct" too, but it's been drowned out by our individual and collective schizophrenia.