On reading further, I have to admit that I'm unsure about my explanation of what was meant by "proposition". I'm a bit confused myself about the theoretical stuff, and the explanation I gave might be wrong. I ought to have been more tentative in my definitions. Could "proposition" perhaps mean each part of the comparative structure (the two things being compared) viewed as, or paraphrased as a whole statement? For example,
(1) sjaldan verða kvistir betri en aðaltré
"seldom are branches better than the trunk"
FIRST PROPOSITION: "branches are good"
SECOND PROPOSITION: "the trunk (of a tree) is less good"
(3) hon er hverri konu fríðari
"she is more beautiful than any [other] woman"
FIRST PROPOSITION: she is beautiful
SECOND PROPOSITION: other women are less beautiful
That seems to relate more closely to the general meaning of proposition (a statement). Maybe someone more knowledgable about linguistic terminology can help.
Also, I was mistaken in using this example, since 'málaefnin' doesn't even come after the 'en'. And a more accurate translation of 'málaefnin' would be "case".
(4) Sýnist þat nú enn, at betri málaefnin hefi ek en þit bræðr.
"It is now still apparent that I have better circumstances than you (have) brothers."
Sorry to add to the confusion! To recap, a noun after the 'en' can be in any case; it's in whatever case it would be according to its context in the sentence (most often nominative in the examples I've seen). But it would be good if we could find some examples to illustrate this properly...