You say that these nouns are 'uncountable', but this is not so. You use the word simplicity when referring to a single simplicity and simplicities is used for two or more simplicities, the same with difficulty and difficulties - they are simply singular and plural nouns.
'the difficulty I have with English is the spelling' or
'the difficulties I have with English are the syntax and the spelling' (more than one difficulty)
The fish question is one of those strange anomolies that appear in the English language to confuse us! These are some exceptions to the normal rule
one fish, two fish
one sheep, two sheep
one deer, two deer
one goose, two geese .... and yet ...
one mongoose, two mongooses
one mouse, two mice
one cactus, two cacti
one hippopotamus, two hippoptami ... and yet ...
one duck billed platypus, two duck billed platypuses