1) its something related to skin and multiskin properties, but i need a bit of research before being able to answer this
2) you have to change the projectileclass property (and/or others related) by modding or commands
3) yes you can! u can put code and textures in the same file alongside with the map (the .dx file) by importing them into the special MyLevel package (anyway this can cause some problems). actually, the different files (.utx, .umx, .dx, .u, .uax, .uxx) have all the same format! moreover, the dx engine detects what content should be found in a file not depending on the extension, but on the folder where it is located!