Field curvature usually is exacerbated at short distances. I am not surprized by your results.
Differences in exposure make comparisons difficult, though.
If I were to judge after the last image pair, I'd say the f/1.2 at f/2 is definitely more contrasty, and possibly sharper, in the center than the f/1.8.
Comparing those fifties can be a frustrating experience, in the sense that drawing definitive conclusions is difficult. Also, the problem is correlating test results with shooting experience. I still don't get it why the 50/2 AI is one of my favourites, while apparently the "weakest" (at large apertures). Then again, I use it generally around f/4...