Mance's, Sandor's and Gregor's fate wasn't entirely clear to me. Was this your intention...

Oh, yes. I like to leave a few people hanging from cliffs...

Now that Tywin is dead, does this mean that Casterly Rock goes to Cersei as his only eligible child? Or to Kevan?

Cersei has the strongest claim. It is not impossible that the Rock will be a bone of contention among the Lannisters, however.

Neither Robb nor the Night's Watch learned that Dreadfort man had attacked Ser Rodrik and his host at Winterfell, and had burned castle and town. Does this then mean that nobody of Ser Rodrik's host could flee to tell the truth about the role of Ramsay Snow?

Most of the leaders of Ser Rodrik's host were slain, but a good many of the common soldiers survived and have doubtless straggled back to their villages and holdfasts, spreading tales as they go. Of course, the situation was confused enough so that the tales may disagree, even with each other...

We were repeatedly told that Ashara Dayne threw herself into the sea. I wonder how this is known for sure. Was her body ever found?

No.

We know that Roose Bolton had already taken Walda Frey to wife before Robb married Jeyne Westerling. Does this then mean that Walder Frey had already planned to ally himself with Bolton to murder Robb before Robb's marriage betrayal, or was his anger towards Robb and his reasoning towards his own family as to why Robb had to be killed more than just a pretext, and the genuine reason for the Red Wedding?

"What if" questions are impossible to answer with any certainty... knowing old Lord Walder's character, it is likely he would have searched for some way to disentangle himself from a losing cause sooner or later, but his desertion would likely have taken a less savage form. The Red Wedding was motivated by his desire to wash out the dishonor that was done him...

As for Bolton, if you reread all his sections carefully, I think you will see a picture of a man keeping all his options open as long as he could... sniffing the wind, covering his tracks, ready to jump either way... even as late as his supper with Jaime at Harrenhal...

Thanks for all the kind words. I'm glad you enjoyed the book.