If I read you correctly, you're running the 1670Z with the same adapter and domeport than Zen sells for the 12mm Zeiss - right?
Not quite - same glass but different mount. This is the Zen DP170-N85-II. This is the 170mm dome for Nauticam m4/3 and Sony NEX systems with a built in 30mm extension. No adapter or further extension is needed with the 1670Z. Phil Rudin started an extensive thread on this dome within this forum.
does the 1670Z focus at every zoom length without a diopter?
Almost certainly yes. The 1 week trip we used these combinations on was primarily wide-angle, so we used the 1670Z for only one dive with pelagics, and as such it was probably used mostly towards the long end of the zoom range. We didn't methodically check at the 16 end. However Nauticam support this lens with their own acrylic 170mm port without need for a diopter and this is equivalent to the glass DP170. I also checked with Ryan Canon from Reef Photo / Zen before purchasing and he confirmed compatibility of the 1670Z without diopter.
How did you fit the zoom gear through the Zen step-up adapter - or do you already have the new one with 77mm inside diameter? Or are you using some kind of Nauticam adapter to the Zen dome?
The beauty of this version of the DP170 is that it doesn't need a step up adapter, and with the 1670Z it doesn't need an additional extension. There is therefore plenty of space for the 1670Z zoom gear.
When it came to using the Tokina 10-17 fisheye with Novoflex Nikon/NEX adapter, space was much tighter when an additional 30mm port extension was added. However there was still just sufficient space to design a custom gear that passed over the Tok barrel with rubber grips removed but still provided clearance within the port extension.
Hope that helps.