1) If you haven't noticed any of the stereotypical signs of overtraining, you probably aren't. (I'm sure the search button can bring these up faster than I can remember them).
2) See above. Also, if you aren't "hurting" anything so far, it seems safe.
3) Depends, some methods hold the weight constant, where you only reach true failure on the last set like you say, if that is the case with something I'm doing. I'll start by increasing the poundage on the last set and work my way back up to the front, once I can handle the new weight on all sets, I start the increase from the last set and work my way back again. Other methods specify changing weight/reps. For example, the 5x5 program I used had you Increase the weight each time, but only hit 5 reps, the only set you failed on was the last one. Interesting concept that I'm going to use again for sure. I've yet to find the "best" routine for me, but I seem to respond pretty decent to all of them so I'm not going to complain.