"mad MAX" (aka Yokel-Motion 1.0)

Q1: Pretty happy with the 501 on the rear, but there are plenty of mid-range Shimano wheels at good prices at the moment: is there anything significantly lighter around the £100 mark?

They all weigh about the same, you'd get a bigger performance gain by switching to a 23mm rim. Since your aeros will be covered by a RalTech, you can use whatever touring/29er rim is going cheap. On-One are still flogging Novatecs at £16.99, hook one of those up to a Rigida Grizzly remembering to move the locknuts around to get a chainline to match the UltraTorque and you'll still have something that's lighter than a real disc