Sedan seats may very well fit but there will be no tilting foward of the backrest. Also, sedan seats do not have the fore/aft travel length of coupe seats. You may as well forget the back seat if sedan seats are installed.
The seats in the CE were different than those in the 300 E. Much wider and deeper side bolsters. The sedan seats would fit, but you will feel the difference.
I don't think the 126 coupe seats would work. The seats are much bigger if I remember correctly. I had a friend with a 560 SEC and seem to remember they were much wider. You also may need to think about the controls for the seat and whether they are compatible.
BenzWorld.org forum is one of the largest Mercedes-Benz owner websites offering the most comprehensive collection of Mercedes-Benz information anywhere in the world. The site includes MB Forums, News, Galleries, Publications, Classifieds, Events and much more!