The 24/1.8 might be worth a look, thanks, but am really targeting 20-21mm range as already have (and love) the 35 Sig.
As for manually focusing the Sig 20, we have a lot of manual focus glass and for the 20, it was a non-starter / not worth the effort (though worth a shot). Just not enough 'pop' in the VF (LV doesn't jive with what we do) and no clear focus indicator pattern. Besides, if I want (or must use) manual focus, I'll get a lens with mechanics suited to it. The Sig's are better than many, but being an AF lens, a far cry from a Zeiss, Voigtlander, Leica, Mamiya 645, Hassy V, etc,.
We've owned the Zeiss 21 in past and very much liked it. While I agree with the listed tests, if the 20 won't focus with any kid of consistency, it's not of much use. Baring some other lenses mentioned, the 21 may be a safe working option until/if a faster, sharper, as reliable performer hits the shelves. To use a car analogy (as 'we' always seem prone to do) if the Ferrari is proving too big a PITA in daily use, no foul in falling back to the Mercedes.
EDIT: Also, given the fixed hood, we were also looking at picking up a Lee SW150 (or equivalent) and some Lee/NiSi, etc., ND filters in addition to the 100m Lees we already have to enable use of the Sig at wider stops (we rarely use the Rok with strobes), so it had to fire on all cylinders to justify the added goodies.
If only Zeiss could create a 20-21mm F1.4/2 sibling to our 135 APO at (ideally) a Milvus price point...
On the Sig 20, great glass, worth a shot as we love the Sig 35/1.4, but such is life. That's what great vendors are for.