Yeah I would target 18-19 degrees at redline and start from there. Yeah you can use the v7/v8 ecu on your exisiting wrx harness, then add in the avcs stuff like you would on a JDM 207 swap. Or at least, It should work like this. The jdm avcs may be different than the usdm avcs which may require some changes be made to the setup, but i would imagine it has been done before!
hopefully somebody else with more knowledge than I will chime in on this.
I understand the visualization idea, but its moot if you have to disable your knock sensor anyways.
Meth is good for safety, I certainly would want to find my MBT prior to adding meth, and then use meth as a safety net. The procedure you described is going to find MBT with meth raising octane levels which can be dangerous if you run out of meth without a good failsafe. So basically you need to do all your tuning with no meth, hit your MBT, back off a little, then add meth and redo fueling to take in the meth. At this point you should be done(at least for the time being)