All "staircase" keyboards are flat, but have contoured keycap profiles, which is the same as on majority of "flat" keyboards (without scissor switches).
Goldtouch keyboards are "fully" tentable.
Keyboards with symmetrically staggered rows are most notably microTron and some older Japanese keyboards; there such modern DIY projects too (one is called Katana IIRC, and bpiphany makes a new generation of Phantom PCB, that should support this).
I believe the general consensus is that a keyboard is "fixed split", when there there's another key or empty space between G and H keys on QWERTY. Thus, I'd put TECK, TypeMatrix and the wireless MS Ergo 4k there.