I can't be absolutely sure (I'm french and I sometimes misunderstand English texts) but rules sounds to answer clearly to your questions...
Page 173: "The chance of fumbling an action roll equals 5% (1/20th) of the chance of failure." So, it is obviously after every bonuses, penalties and skill modifications have been done... Page 175: "Special Success (roll < or equal to 20% of target) / Critical Success (roll < or equal to 5% of target)". The target number is the modified skill, not the skill as written on the character sheet. Page 177: "Circumstantial modifiers should be applied after the base chance has already been adjusted based upon action difficulty, as described above." So, to my mind, the answers are always (a) in your examples above. Note that, here again, the rules are made to simplify things: "As a rule of thumb, if circumstantial modifiers would modify a base chance up or down by more than 30%, shift the difficulty of the roll up or down one level instead." Which means that an easy action with + 30 and + 20 bonuses just becomes an automatic action and that a difficult action with + 30 and + 20 bonuses just becomes an average action without bonus. Which gives exactly the same result than those given by the calculation, actually.