We're talking about internal headsets here I think, the Monty Internal headset is chuffing light (100g including top cap etc), definitely lighter than the Onza, but not really worth changing unless you are on some crazy weight mission.
International standard for MTBs is 9mm (which is what the Trialtechs are). The Echo bolts are 10mm which is where the issue lies...
In the past we have seen people remove a bit of meat from the dropouts on the fork, that worked perfectly. As long as it's neat it won't void the warranty either, Dave is aware of the problems with Echo hubs and understands that it may need to be done.
Yeah, that was a 2.2 model: http://www.tartybikes.co.uk/media/kdays09/21.jpg (Yeah, that's an Atomz, but same setup on his Coust)
A 2.4'll never fit with 18:16, but I have seen it run (with rubbing under hard pedalling) on 18:15.
Keep an eye on those caliper bolts dude... They will work loose very fast as there are so many spacers.
Personally I would sacrifice a bit of 'frame squishing' to have a few less spacers in there, or put the majority of the spacers on the drive side if you can.
Pretty sure Rob had a Monty hub in his - you can squeeze the frame in a bit and it should be fine, but ideally a 'proper' 116mm hub is the best solution...