From my experience...
1) Live concerts are about 90-120 minutes, depends on set list and so on
2) Depends on location. In big cities (like Tokyo, Osaka) concerts will be sold out fast (though tickets sale is staggered, but as soon as general sale starts, all remaining tickets will be available). In small cities, there may be a better chance getting a ticket.
3) If you plan to travel around Japan I suggest the Japan Rail Pass (maybe only a regional pass if you stay in a certain area). If you stay only in Tokyo, you better get a Suica and check google which route to use. Suica has the advantage you don't need to buy single tickets (it's a prepaid card and it will be charged the correct price)
4) Depends on the club. I was one time at budokan and I couldn't see that much from the last row (but it was still great to see SCANDAL live!). But at smaller clubs even the last row is good enough (and as non-asian you might have the advantage of being taller than the rest). I'm going to the concert in Takamatsu and the "Festhalle" can host 950 people, so I will have a very good feeling (well, I won't be in the last row as my entrance number is 285).