Re: travelling through poland
I have travelled through Poland several times, and personally, I would visit Wroclaw, Krakow and Warsaw. Warsaw and Krakow have more history than Poznan, though Poznan is a beautiful city. Of course if you want to visit the sea, then Gdansk might be a better option. I think connections to Vilnius will also be better through Warsaw. There are direct bus and train routs from Warsaw, (no Belarus), though depending on when you are there, might not be every day (every second day).