I think that the parts in Rome and Helsinki are so full of fingerprints from Benigni and Kaurismäki. They must have been part of the writing/re-writing.
My guess is: A short idea by Jarmusch and the whole setup by B and K. They are both friends of Jarmusch, so that, I guess, he wouldn´t have to credit them for the writing.
I think just about anyone who has seen any film by Aki Kaurismäki connects this Helsinki segment to it, if only just because of Kari Väänänen and the somewhat gloomy and depressing image of Helsinki that comes through. But the dialogue itself is not really that laconic and it's also filled with curses and other stuff that I don't see have any connection to Kaurismäki's usual style with dialogue. What did you mean exactly?
Well, you might have a point there. I haven´t considered the conversations in the Helsinki-scene in this matter. It´s more the slow melancholic way of acting from the main characters, that reminds me of Kaurismäki. The actors do fill out the room in a certain way. Like the young Kati Outinen did in Kaurismäki´s films. Very few directors have this talent of filling out time in such simple and thoughtful way with everyday-cahracters - and at the same time make it something special. Of course Kari Väänänen gives the rest. - He was/is by the way one of my favorites, both with Kaurismäki and other directors, sorry that he didn´t stay among us a little bit longer. It´s not that I don´t think Jarmusch was able to do this on his own, he certainly was, I just still have the feeling that the 2 had a word to say...
I think jessonderskov has confused Kari Väänänen and Matti Pellonpää. Both are cast in the Helsinki scene, Pellonpää as the driver and Väänänen as one of the passengers. As far as the information I found in the Wikipedia is correct, Väänänen is still alive. Pellonpää, on the other hand, who was one of the greatest actors in the world (see Arvottomat to see his great acting), has been dead since 1995.