Why is ‘boring’ suggested but not accepted?
Diflas can mean boring but when in this instance it better translates as miserable because you are describing the weather.
Hints are not "suggestions", and they're only very weakly context-specific - they'll be shown on any sentence containing a given word whether that particular hint makes sense for that word in that sentence or not.
Why is diflas lenited?
yn (here: the 'n of hi'n) lenites nouns and adjectives used predicatively (to describe the subject) after the verb "to be", as far as I know. (But not verbal nouns.)