Hi Zoolean,
For (a), I just run the 'range' version with extra textures and it will stop playing a sound on new textures as you need to add 3x sound variations for each new texture...if you dont want to get that then just pick some existing ones already and copy and rename to match the last 4 letters of the new texture names as there is no ability I am aware of to check for missing sound files before playing a sound in CopperCube...
For (b), cant see any problems...did you add a different plant texture to the standard ones ? if so you need to add 3x sounds and place them inside the 'sounds' folder that comes with the files and must be in the same directory as the 'exe' also.
Let me know if this fixes your problems...