In my experience that works for some parts that are clearly NGK, Denso, NTN, etc. For the OCVs I haven't found that to be the case. It's not impossible, but I haven't seen where people found the manufacturer for the Subaru OCVs and bought from them instead.
I don't see that. Maxing out fuel correction is +-15, you are within +-5, which is quite good.
It may take more time/driving for the correction to move further away from 0.
I don't have any one that is looking for a tan drivers seat, so you're safe there. I have a little touch of surface rust on mine, but should be easy to wire brush off. I'll see what it takes to remove the frame rails, you probably don't want the split seat cover that's on it right now.
I think some of those "Sti" versions were more about the bushings than the metal. I agree with KZJonny, if they look the same, I would just keep what you have depending on the bushings.
Nice, I teach Computer Science with some engineering department neighbors. They are building a baja cart program that I really wish I had the time to help with.
I looked through the Cusco guides to see if I could find an answer for you, half of them didn't have torque specs. It's into aluminum; so snug, but not too much...
Yes, for the Legacy GT 05-06 is 4.111 and 07-09 is 3.900 for the rear diff.
You can check on cars101.com or https://rallispec.com/downloads/Transmission ID Chart_Public.pdf
If the WRX diff is different for the axles, get a 05-06 LGT rear diff, it's still 4.111 and matches your axles.
If it's been a year, I'll be surprised if the rear diff doesn't match and the transmission survived this long.
I would guess you'll want/need a new pivot ball and fork from a GT to match up with your Exedy clutch.