Reminder : I'm not a coder and in addition I don't speak english very well
In JRR skins collection and I guess most of other skins, a big part of panel code is NOT logged. Many parts of a panel's code which are not customized do not require to be logged (we just have to keep the main tree structure, however we can expurgate all elements we are not focusing), the lotro client will consider all missing parts as default code. (And it works the same way with picture elements. All pictures files not logged in the skindefinition file are considered as default pictures).
So, probably, you should always employ the skinning source element codes as base code and then apply custom skin element on it to mimic the game mechanics of rendering and then you should get all the panel elements correctly displayed.
Concerning pictures, if an element is to short in width or heigh or both compared to the surface it is intended to fill, the lotro client duplicates it in order to fill all the remaining space.
If, by mistake, the same assertid is present many times in a skindefinition file, the first occurance (from the top to the bottom) is considered as the good one.
I hope this can help.