Wow maba it's looks like you have a great starting point! The BF issue does seem a little bit odd though, with the 7-8% difference between the calipers and your scale. I don't have a BF measuring scale though, so I really have no clue what mine is right now. Maybe it's in the middle of those two readings, somewhere around 22%? (Which is still awesome!)
For back flexibility definitely start doing yoga. There are so many wonderful back stretching and twisting poses that will help a lot! I know she's a little cheesy, but I do really love Denise Austen's Yoga Body Burn DVD, it's a great workout even compared to the classes I've gone to.
I'd also recommend doing your weight training as circuit training, which will help with your bicep strength and aerobic fitness at the same time. I like to do full body compound movements in quick succession, ie: Squats with an over head press with 10-15 pound hand weights (weights in hands, ready for the overhead press, squat down, then do your overhead press as you raise back up), then after a set of those immediately into lunges with a bicep curl, then immediately into an ab hold/exercise of some sort, then immediately into hay balers (look that one up) with one weight, etc. Keeps your heart rate up, and uses your full range of motion with added weight. Oh and keep doing the assisted pull-ups and dips, and work on using less and less assistance (it's hard, I know!!! I'm down to about 40 pounds of assistance myself). I like to do maybe 2-3 circuits of 6-8 exercises for a total of 30 minutes on weight days, and I'm always tuckered out by the end