Ok, lets take it point by point...
- Some questions of NBME are ofcourse super tough and we don't have clue about them, but so does UW and the real exam! I feel we can't hang on to a few tough questions. Once the question has passed, move over. NBMEs are not that bad as you are thinking! They are not meant to break our onfidence and all! My point being we will get super tough questions occasionally and just say to yourself "Whatever!!" and move on to the next one. The genereal perception is that NBMEs are tougher than real exam but as I said, NBME are meant to predict our scores and not meant as a learning tool.
- The real exam, as I felt was in mid-way between UW and NBME, maybe slightly skewed more towards UW. But honestly, 8 - 9 hours in there...over 300 questions...you can only guess the closeness of the real one to UW and NBME. As with every experience that I have read, I also felt crappy after my exams! So be prepared for that!
- I did get 2 questions similar in step 1 and 1 in CK from the NBME ( I gave only 1 NBME for both steps so number of similar questions may be higher for people who gave multiple NBMEs) But, the question stem was same and a different thing was asked in 2 of them and one was the same one. I read somewhere (can't recall where) that it might be that the questions that are repeated might not be carrying any marks/experimental questions/pure luck blah blah blah. You can do the offline NBMEs but I didn't do it so can't really say how good or a bad idea it is just for the purpose you are mentioning about. It would never hurt, but you have to decide upon how much time you got!
Having said these things...I would advise you not to worry about NBMEs being like a cheat-code! The fact remains simple, practice practice practice!! Whether NBMEs or UW or Kaplan!
(depending upon the time ofcourse!)
Good luck !