[William Cloud Hicklin]

I did have the notion that (b) and (c) could perhaps be reconciled by, as you say, having Bilbo translate specifically Dunedainic works originally composed in Sindarin; my conceit was that the Elves don't write "history" as a mortal mind would understand it (however, the existence of the Annals makes this idea problematical, unless those too are explained as Numenorean retro-chronicles like much of the ASC.)

However, that leaves no room for (a), the Eriol/Aelfwine vector. I suppose it's worth noting that the passages in the Prologue concerning Translations From the Elvish were added in the Revised Edition (1965), and might therefore be considered to represent Tolkien's latest written thought on the matter.