First of all, you are close from a working stand point. The Lutheran view is that you are saved by grace, but also that your love for God will lead you to do His will. If you say you love God, but don't do His will, do you really love Him? In other words, you have to walk the talk. The works don't save you, but if you really love God you will want to do His will.
Second, the JDDF is a flawed document. The reason most of the more conservative Lutheran bodies didn't sign it is because after the appendix came out, there was very little to sign. While the prior document had listed many points in common, the appendix basically said “no, we don't think so”. The current pope was pretty upset about how that worked out, and I wouldn't be surprised if something is in the works right now to modify that. I do know that the LCMS has sent a representative to the Vatican, possibly for just that sort of thing.
And lastly, the LWF is a bit like the Anglican church right now. Parts of it are pretty good, most of it is way off the rails. Think women priests marrying their gay lovers and talking about the holy “sacrament” of abortion type of thing.