Actually not at all. You just can NOT mount the J-12 on the Contax IIa and IIIa.
Not because of the rear glass to shutter curtains distance but because of the upper shutter curtain additional housing in those cameras lens chamber, compared with the Contax II and III (and the Kievs). In other words, it's just impossible.
The W-Nikkor-C 35/2.5 is a better lens than the J-12 (has less vignetting and is sharper corner-to-corner). A mint late black (improved coatings) W-Nikkor-C 35/2.5 is one of the best 35 ever made and a must-have if you own a compatible camera body. It will mount with no problem on all cameras sporting a Contax-Nikon RF mount. On the Contax cameras it will focus just fine.