I've already reverted it.  I realized that I use tab more than I thought I did, and that tab as an esc key was probably inferior to AltGr+t.  In that case, the only advantage were the paren positions, which weren't really that bad in the first place.

I'll try experimenting with altgr+[single character] spots to put [].

Tab is a dangerous key to fiddle with in my experience. Doesn't play well with PKL for sure, and XKB would give you trouble as well I'm sure. As you touch upon, it's expected to be where it is, and changing what's taken for granted can be tough.

I'm quite happy with the Caps+Q Esc; while technically two keys I hit it as a chord using pinky and ring finger. Although these are weak and interdependent, it works fine because the chord is neat.

I don't think you mean evdev (look it up) but the keycode component of XKB. ;) It happens to be in a file named 'evdev', but there are other files with that name too.

As you may know, I use that for the Angle/Wide mods – precisely because it's layout independent and semi-low-level.