In ‘Haud difficilis est ars volandi’ is ‘difficilis’ fem. nom. sing. in agreement with ‘ars’?
Yes indeed - this is one of those proverbs where the trick is separating subject from predicate - and strangely for English speakers at least, the predicate comes first:
Haud difficilis (predicate)
est
ars volandi (subject).
As you would expect the predicate adjective (difficilis) agrees with the subject noun (ars). 