Do you overpack?

I think you already got your answer about the 1" fork, your builder said no. You're going to have to put in some research to find someone that will make one for you. Retrofit is out unless the existing fork is an overbuilt unicrown fork. Most forks built for rim brakes are very marginal for disc, it's not unlikely that it would fold under heavy braking.

The Ritchey fork has Axle-Crown: 393mm. Good news is it has 50mm rake, because that A-C is likely to push the front of the bike up and reduce the head tube angle.

I'm a little surprised I'm the only one who thinks you shouldn't put this much effort into major modifications on a $1500 bike. Does your framebuilder work for free?