1) For audaxing/very light touring do we really need a solid axle (for obvious reasons I'd prefer a QR)?
I'd say not. But the two of us weigh under 19 stone. If your team is significantly bigger, that might not be the right answer.
2) Why are all the rear wheels on the Thorn site 26"? Are 700c rear tandem wheels a no-no?
Robin Thorn likes 26" wheels, and you might have noticed, most of the Thorn bikes are 26" wheeled. One of the brochures used to give a reasoned explanation of the preference for 26" wheels. Not being anywhere near 6' tall, I think a 26" wheel looks in proportion to me and don't have a problem with it.
Just pick up the phone and talk to them. I expect they'd build you any wheel you wanted, although if you don't know exactly what you want, you may be helped along the road to a decision.
3)What would the most sensible rim/hub/spoke combo be for general use (think audax as the main use)?
That's all way beyond my competance, I'm afraid. In fact I'm still trying to work out the answer to the rim question. But we're using a Rohloff hub with 32 stainless DT butted spokes (and a hollow axle). Over 4000 miles on one (26") wheel and getting near 4000 on another, and the wheels have never broken a spoke and never needed me to go near them with a spoke key. So they are strong enough and well built enough in 26" form even with 32 spokes.
We use our bike for similar things to your plan. Day trips of up to 100 miles or more, usually carrying one of those enormous Abus Granit 'D' locks as well as a tool kit, 2 spare tubes, 5 bottles of drink, spare clothing etc.
