I've been running tubeless for several years but not heavily loaded touring. Things that I've have learnt are: (i) Small size variations make some tyre/rim combinations easier to mount than others and a good combination means that the tyre will stay locked on the rim even if deflated (if the tyre is a loose fit then add another layer of two of tubeless tape). (ii) Thorn and similar small holes go unnoticed and cuts are the big hazard. About a year ago I had a nasty encounter with a small piece of flint which made a cut (see photo) that neither the sealant nor one of the tubeless repair rubber worms could handle so I had to fit a tube. The tyre was a Gravelking which is lightweight and rolls OK but isn't very tough (and the Plus version with an extra layer of fabric isn't offered in UK in the 650b size).

I would expect that rims and tyres that are sold for rougher gravel cycling will also be suitable for loaded touring with, for tyres, cut resistance taking precedence over knobbly tread. I recently fitted a pair of these https://www.bananaindustries.co.uk/products/irc-boken-plus-tlr-folding-650b-27-5-x-42mm (also available in 650b 47mm) and have found that they roll quite nicely while the "plus" in the name indicates an extra layer of fabric. IRC is a Japanese company and it's possible that they would be available in China.

In addition to a tubeless repair kit also carry a small bottle or two of tubeless sealant so, if you do get a hole with sealant loss then you can top up the sealant (you will also need a valve core removal tool - dust capes are available with the appropriate hole). Not all sealants are equal. Those containing some bigger lumps stand a better chance of sealing cuts.

Your terminology is good enough to make yourself understood, Andy. I would say that in the theory, the four motorcycles* you describe, whether oversteering, understeering, or easy to hold in a steady-state turn, are all predictable after the rider acquires familiarity with them, and therefore are safe, some more than others. Precisely how safe would depend on how much a large road irregularity will upset each one and the level of skill required to recover from an upset.

I'll be following your tour with great interest..

*Lucky you!

I am intentionally ignorant of tubeless.  Can't help.

I typically run about 67 to 75 percent as much pressure in my front tire as in my rear, as the rear is where most of the weight is.  That has always worked well for me, both touring with a heavy load and unladen.

You said you found a wheel builder that can work with Rohloff hubs.  I have no clue how old your hub is.  Is it old enough that you lack the reinforcing rings?
https://www.rohloff.de/en/company/news/news/flange-support-rings

The wheel builder will of course need the right length of spokes.  Once you decide on rims, you should communicate with the builder in advance to make sure they have what they need for spokes and nipples.  If your bike was built by Thorn, the rear hub is likely 32 spoke, but some Rohloffs are 36 spoke.  (Mine is 36, I built up my wheels, bought the 36 spoke hub I wanted for heavy touring.) 

Sapim Polyax nipples work better with generic rims when fitted to a Rohloff.  The nipples come in different lengths.
https://www.sapim.be/nipples/design/polyax

I assume you know the above (rings, spokes, nipples, etc.), but I mention it just in case you did not.

Good luck on your transition to tubeless.

Paraphrasing……
If equal sized tyres at equal pressure give neutral handling then changing to a wider, softer walled front tyre at a low pressure and a narrower, stiffer walled rear tyre at a higher pressure will reduce understeer (and may make the bike too responsive).
Is that correct?

No. "Neutral handling" is a dangerous misconception -- a neutral bike would be a lethal bike because you don't know which way it will break.  What you mean to say is "If equal sized tyres at equal pressure give PREDICTABLE, SAFE REACTIONS TO STEERING INPUTS, then" etc.

It is worth emphasizing the important implication, that roadholding/handling aren't just theoretical results of tyre choices, but in the first instance a matter of the rider's safety and security on the bike, which become more important on foreign roads of unknown quality, at higher speeds (25mph is more than enough to do you serious harm), and as the load on the bike rises as it normally does in touring. An understeering bike is safer than an oversteering bike (and a neutral bike is NOT WANTED anywhere except at the very top of road racing).

My cycle speeds are much lower than Andre’s, max 25mph (40kmh), normally 12 - 16mph,  and I rarely corner fast.

Thanks for the giggle. I actually ride slower than you do, on average 15kph/10mph, but I'm rarely on the level for any distance, so on the downhills I can get up to 55kph, though I'm now of an age where a broken hip could easily be a death warrant, and will almost certainly be the end of cycling, so every year I back off 5kph on the steepest descents, and stay an inch or two further away from the edge of the tarmac, which is also generally the edge of the deep ditch beside the lane.

My logic is that the softer / larger front tyre will give a little suspension over bumps while the tougher Marathon Plus on the rear will be more puncture resistant.

In my experience the Schwalbe Big Apple and the Schwalbe Marathon Plus (both with the best compound and the toughest anti-intrusion band) are equally puncture proof, each pair succumbing to two punctures over the course of about 11,000km. In theory, the Plus should be more puncture-resistant because of that ghetto-style anti-broken bottle sidewall where the Big Apple has a soft, thin, very flexible skein that looks like it could be broken through by exhaling near it.

Could Andre expand on his second to last paragraph, copied below, as I seem to be changing my bike in the opposite way!

‘If we're talking about touring with any kind of a load, even just a saddle bag, never mind the loads we see the real transcontinental tourers on the forum carry, it seems to me obvious the fat tyre had better go on the back’

I think you've not only answered yourself already...

I am interested in a softer 2.00” tyre on the front with the existing 1.75 Marathon Plus on the rear for touring on roads that may be rough in Sri Lanka this coming winter.  My all up weight with food and water will be 105 - 110kg.
My logic is that the softer / larger front tyre will give a little suspension over bumps while the tougher Marathon Plus on the rear will be more puncture resistant - and will be dragged over any bumps.  Repairing a rear wheel puncture is always a bit more work than a front one and rear wheel punctures seem more frequent, hence the Marathon Plus on the rear.
The slightly larger / softer front tyre should also be better at going over any sand I hit, while the heavier loaded and narrower rear tyre will did a groove and slow me down and help me stay upright. 

... but also explained why I referred to your experience.

You may remember we arrived here via me explaining that this sort of engineering thought exercise is an art. It's actually worse than that in this particular instance because tyres, whether on cars or or bikes, are infuriatingly non-intuitive in response, and frustratingly resistant to reduction to algorithms. However, there's a small relief here because we can just assume that all the other design factors on your bike are near-optimal, because the bike is a Thorn, known to be properly designed as a conservative tourer. So my first approximation, in a theoretical discussion that newbies may read and try to apply, was aimed at a standard, normative case, an answer that would endanger no one.

Add your bike load, and your experience, and we now have another condition, a loaded touring setup which you found satisfactory. The fat, low pressure tyre at the front has plenty of grip and a large contact patch to turn the greater weight, but will be slowed a little by it. The narrow rear tyre, even with greater pressure, will also squash out a bit more under the luggage mass and slow down any tendency to dangerous oversteer further. Looks to me like the load actively countered the "wrong way round" fitment. Also, your conservative incremental style of developing your bike is inherently safer than making large changes.

Something else to consider: A load in panniers and a rack bag provides surface towards the back of the bike for sidewinds to push against. This can be a good thing because bike stability is best served by the Center of (Aerodynamic) Pressure behind the Centre of Gravity of the bike with luggage and rider in place.

Ryde's Rival and Edge series are tubeless-compatible. (Note that the Andra series traditionally popular in the bicycle-touring space, is not.)

If I was getting so many punctures from truck-tire wires, I’d diversify my route away from main highways.