Yes, all of the yellow bolts come off, as well as the pump bolts. There will be two more behind the pump that have to come off. Also he forgot to mark one bolt under the shift solenoid at the top left of the channel plate. I figured that out on my own.
What year is your car? and how many miles? Ever had a rebuild? I'm wondering if you have the hardened splines on your 4th clutch hub. I'm assuming not since the hub and pcs design got changed around the same time. And if it's fine, I wouldn't suggest removing the channel plate just to add two springs in the 4th accumulator. It's a decent amount of extra work and money for something like that on your first time.
If you're doing the boost valve, you have to remove the valve body from the channel plate. This means exposing 4 check balls. It can be done while still in the car, on the channel plate, but it's easier on a table. Is it bad or need to be replaced or something?
"If it turns out that I can do the boost valve and pressure regulator spring without removing the boost valve" < I'm assuming you meant valve body or channel plate at the last part? You will have to remove the valve body. The channel plate doesn't have to come out, but you'd be working with it on its side since its bolted to the trans.
So basically the channel plate only has to come off for the 4th accumulator, and 4th clutch hub. I would only remove it for doing the 4th clutch hub if you need it(or it should be done) Mine did so I did the shift kit while I was there.