To 1) Yes. Either create a unit on map init or simply put a preplaced unit on your map will load the unit's model and all of it's skills.
To 2) Should also work. Though I never needed to preload items.
Preloading will, of course, increase the loading time of your map. Depending on how much you preload that can have a real impact. One of my maps had (preloaded 8 heroes) ~40 seconds loading time without preloading (but lags upon first casting of spells) ~17 seconds.
They deleted one of my posts one time so I was trolling him on a help post I put up because I am evil I guess and bored and the guy handled it well I loved his attitude Atari has a good social media presence even if they do not have many games.