Croy,
The Hy addition for GTAW is pretty common in Europe for super alloys like Inco 718. It enhanses wetting at the edges, has better cleaning action in the arc plasma, and increases heat input per amp. The Hy additions are more rare in the U.S. but as mentioned earlier, some of the majors (Pratt, GE, Allison, etc) have used this mixture with good results in procedure quals and production. There shoud be plenty of data available on Inco alloys and various GTAW shield gas mixtures. Don't know if it would be an improvement over your Helium mix.
Filler metal selection for an application like yours is above my paygrade.... Sorry.
See ASM Metals Handbook Vol 6
Here is an interesting link
http://www.stainless-steel-world.net/pdf/11012.pdfI'm sure inco alloys has data for you as well.
Pulsed GTAW is an excellent idea for reducing heat input on semi automatic and rotated projects, Sciaky Accu-pulse is a good example. Pulsed GTAW loses effectiveness when manually operated simply because an operator can hesitate or travel more slowly than the plan and more heat just keeps pouring in.
Who ever came up with the fabrication sequence that has been put in your lap should be kicked---- hard. :)