Sadly, you are right. My first 35 FE Sonnar had one obviously soft corner, but thankfully the second is wonderful. My 28-70 kit lens is very good but not perfect. The bottom left extreme corner is a little behind the others, but its so good overall I'm not that bothered. Truth be told, if you test most kit lenses you will find some issues, but sample various in top primes is a little more frustrating.
As for good wide primes, the best I have available (and hardly practical for most) is a 24 Summilux asph. By f5.6 is 'good' but by f8 it is very good indeed and would allow for beautiful prints. The corners are very crisp by this aperture, so for landscapes its fine.
The CV 35 1.2 II is superb. Absolutely brilliant. By f5.6 it overtakes the 35 FE Sonnar at the edges and at f8 is immaculate across the frame. The Sonnar, however, is better at f2.8 on the edges and in the corners than the CV 35 1.2 is at f4, however.
I think everyone is waiting for Zeiss to release a 24mm prime, or perhaps a 21mm. A 16-28 or similar would be great, but no sign on the near horizon it seems.