Everything about track status can be retrieved or set with the function GetSetMediaTrackInfo(). Are you looking at the generated reaper_plugin_functions.h file? (Actions->[developer] Write C++ API functions header) That file is fairly well documented.
For sends you can manipulate the parameters of existing sends using GetSetTrackSendInfo(). If you want to create/delete sends, you'll need to use GetSetObjectState() which returns a string that needs to be parsed. Make sure to free the returned string with FreeHeapPtr() as well.
Feel free to use the source for the SWS extension as a reference, there are examples of all these functions' use and more. Browse the code
here.
Good luck!
Tim