Hey Welcome to the site!
Now, don't be put off by the length of this thread but your questions - which don't really have any solid answers, only speculative ones - are being discussed here: http://www.thedarktower.org/palaver/...ead.php?t=1778
On the last two pages of the thread I've posted a summary of the topics that have been discussed so far and your questions are pretty common discussion themes
As for "why Roland?" I think that's because of his heritage - he is the last Gunslinger and therefore it's his responsibility to perform the quest as Gan sees fit.