Bjorn Is The thief ! 

Okay, so I have to say the scale , as he tells his tale he is detailed but forgets to mention that he himself leaves the chamber which gives him opportunity as all the jarls left and most certainly would have been observed and had no time to commit the act or hide it, 

 let alone anyplace to hide the sword outside of their sword sheaths; which could be checked easily enough.

scald

In my opinion, it's the scald since there are more than 4 inconsistencies in what he has written : -

1) He says there were 6 Jarls --- but names only 5 at the start. (Ulf is mentioned first, but there's a speech by Einar) 

2) He mentions Bjor raising a horn --- but draining a goblet. 

3) He says Ivar can't stand --- but then mentions that Ivar sat down.

4) He says Loki is Ragnar's elder brother --- then states that Loki is younger 

5) He first says Bjorn is strongest --- then says it's Einar 

6) He states that Einar had his hand on Loki, then mentions him slapping his palms on the table, and finally directly removing it off Loki's shoulder. (That does not make sense unless Einar has more than 2 hands) 

(I can't comment on what the Jarls said, though, since I'm not too well versed in the lore) 

"Participants who are first to leave the right answers will get:"

This doesnt make sense .

You can only have one particpant  who is first to leave the correct answer.

There is only one  right answer, so participants cannot leave the right answers.

I am sure I read something different earlier.

Did they change the text?

'X' Participants who are first to leave the right answers will get...... does make sense.