I rode 78 miles yesterday to the South coast and back on the Sherpa to check it all out - great until the last 20 miles when the gearing went wrong and I could not use the big chainwheel.   Looking at it today the ‘Strong and Light’ 42/34/24 chainwheel set I had fitted had somehow gone eccentric, with about 1/4 inch run out of the big cog.  I had been powering up a few steep hills, but it seemed it was neither Strong nor Light and had lasted only 300 miles from new.  An old Shimano CT92 was hiding on the shelf so I have fitted that.  It must date from the 1990s but has had little use and the alloy web from the cranks holds the sprockets much better than the design of the failed crank.
The Conquest was comfortable over Somerset and Devon’s potholes, with 1.75 front tyre and 1.5 rear tyres giving some additional cushioning so I will stick with that.
I rode without mudguards and discovered how much their loss slowed me down as I seemed to be constantly braking when riding through shallow flooded sections of roadway that I would have whizzed through if the mudguards had been on.   So bikes with mudguards are faster!

I use the S&S Backpack case.  More info here.
https://thorncyclesforum.co.uk/index.php?topic=13407.0

My bike is a 590M size.

Once you have successfully packed it, when you unpack it take many photos so that next time you need to pack it, you know exactly what works for you.

I do not recall if I mentioned this earlier, but my Nomad Mk II and the case exceed allowable luggage weights for most or maybe all airlines.  I use a luggage scale to make sure I am in compliance.  The rear rack will not fit in the case with the bike, the rear rack and some other heavy items are removed from the case to get the weight do an allowable amount.  But my tool kit might include a knife, so the toolkit is checked, not carried on.

My past trips, I checked two bags, the S&S Backpack case and another bag because I had enough camping gear that it exceeded carry on luggage volume.  So, it was easy for me to put some of the excess weight in the other checked bag.

It is my experience that airlines are much more stringent on weight but a case that is slightly oversize often flies without an oversize charge.  I have never seen a ticketing agent pull out any form of measuring tool.  My point is that if you get a case that is slightly oversize, you might get lucky.  For example Ritchey Break Away cases exceed the commonly used size criteria by a small amount but they rarely incur oversize fees.  But I do not know if a Nomad Mk II would fit in a Ritchey case, I use that as an example.

Luggage scales will pay for themselves with one saved overweight fee.

I weigh between 80 and 85kg.

Have never broken a spring or a Brooks rail, but I am aware that others have.  I think Dan has.

Before I decided which to take, a B17 or a Conquest, I would decide which one is more comfortable.  The B17 is a bit wider and flatter in back.  Both have parts that can break, but the shape is different and I think the feel on long days in the saddle is a more important factor to consider than the potential of a broken part.

If I was going to ride more of an upright position, I find a B17 feels pretty good.  The Conquest and Brooks Pro have a nearly identical shape.  I find that when I use the drops on drop bars, the B17 (or its sprung equivalent, the Flyer) does not feel right for me, for using the drops I prefer the shape of a Pro or Conquest.

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I recently put a Redshift suspension seatpost on my light touring bike.  That bike has tires that are 37mm wide, so it can transmit the bumps to my hands and bum more than bikes with wider tires.  The Conquest has rails that are far from horizontal, the seatpost did not have enough range of seat angle to use the Conquest, so I put a Pro on that bike.  So far I have only about 25 miles (~40 km) of riding with it, but so far I like that suspension seatpost.  Their instructions gave a setting by weight for "comfort" or "performance"  I set mine up for performance and am quite happy with that setting. 
https://bikepacking.at/media/pdf/a2/29/0b/Instructions-ShockStop-Seatpost-Rev-1.pdf

The Redshift is not cheap.  I got a 25 percent discount on a limited time sale.

If you consider a suspension seatpost, Dan has much more experience than I do.

Thanks for the clarification - I wondered if you had added 25ml (or thereabouts - I agree that 12.5ml is somewhat precise). One approach to deciding if you need to add more oil to compensate for that which escaped is to drain the oil in the hub and see how much is there (then put the oil back in the hub). If less than 12.5ml then add some more.

It maybe entirely unrelated but I twice snapped the seat post bolt on my Conquest, fitted to a MK1 Nomad. I weigh 80kgs and rode my Nomad with vigour but certainly not to any extreme level. I’m minded to think over tightening the said bolt may have caused the failure but I couldn’t say for certain.