Hi Scott and welcome to the forum.
Let me first say that drywall and greenboard weigh essentially the same. CBU is heavier, but can also be used on the ceiling if you increase your screw density during installation.
If the bricks are loose, then by all means take them out instead of shaving them down. It'll save time and you'll take out some of your day-to-day frustrations by hitting something.
I'm not a big fan of pine plywood on the floor of anything, since it is very sensitive to moisture.
To save you some time and more frustration, I'll direct you to the Liberry where you'll find the Shower Construction threads. If there ever was a project that could use the Kerdi system, yours is it. No blocking is necessary and you can use regular drywall - it'll also take you about 3-4 hours to make the shower ready for tile, whereas you'll be spending days prepping it the old way. Read up and come back with any questions you might have.
Outside the shower area, you can easily meet up with the floor in adjacent areas. My suggestion would be to use 1/2" plywood, then Schluter Ditra over it. The tile, with thinset, is applied over that. I'm assuming your Interceramic tile is 5/16" thick, which is their norm. With all three products together, you'll be right at 1".
I'm a big fan of mud, but if it comes down to meeting established floor heights - Ditra is usually the way to go. It's much easier to install than CBU or any board for that matter.
Lastly, lose those two books and invest $23.95 in John's. He's no comedian

and if you can get past the pages with his pictures, you'll find a wealth of knowledge about completing your project - including a whole section on the Kerdi shower.