Swetting off fat doesn't work, it only makes you lighter because your dehydrating yourself but as soon as you drink again your weight will go back up. Boxers do this to try get there weight down for when they weigh-in for a fight. I'd go with shorts all the way - No chafeing!!
not really any harm in practising running in trousers, personally i prefer it. Also id practise running in wet kit, sounds stupid practising being wet and cold but the odd run in wet, muddy gear just gives you an idea what your in for.