Hello jdh020409,

I really appreciate your feedback! As for your questions, my answers as follows:

Bloodthirster is a very expensive item to build compared to Youmuu's Ghostblade, (3700 gold to 3200 gold), however, Bloodthirster only gives 15 more AD than Youmuu's Ghostblade. In terms of combat stats, Bloodthirster only gives 75 AD, most of its value came from the immense lifesteal and sustain it provide for better teamfighting, which is not something Fiora really needs, as she is a splitpusher.

Youmuu's Ghostblade however, gives a 6-second buff which allows her to easily catch up to enemies if they try to stop her splitpush, or run away if they send like 3 people to stop her. Youmuu's is a more flexible item to build in terms of this. Plus, the 60 AD, 20 Armor Penetration and the CDR really boosts Fiora's kit.

But Bloodthirster is still a viable item to build if you are really ahead and wish to build more damage but still help out your team, you may choose to build it third item or forth item, because it makes you much harder to kill, but you must really be aware of your positioning.

As for your second question, I believe that Maw of Malmortius is an early game item, which means that it is effective only if it is built first or second item due to the high base value and low scaling of the shield. However, Ravenous Hydra and Black Cleaver/Trinity Force are the core items of Fiora, building Maw of Malmortius or Hexdrinker delays her power spike by a lot. Therefore, I have listed Maw of Malmortius as a mid-game offensive item, you can choose to buy it third item against AP heavy teams, as it gives decent MR and a shield which is still relevent mid game, and has more sustain if you are low health.

But even though it is a cheaper item than Mercurial Scimitar, you don't need to build the entire item for it to be effective, as you only require the QSS part. While for Maw of Malmortius, Hexdrinker does not grant such a useful active and is usually built early in the game such as during the laning phase.

While Mercurial Scimitar gives 5 less Magic Resistance than Maw of Malmortius, it also grants an active that can cleanse CC as well as a moment speed buff, plus it gives 10 more AD than Maw of Malmortius and slightly more lifesteal.

However, I would not usually build Mercurial Scimitar straight up if I want magic resistance. Against AP heavy teams, I would often go Mercury Treads, build my three damaging items, which are usually Ravenous Hydra, Black Cleaver and Youmuu's Ghostblade, I would go QSS and buy the MR component of the GA, it would give me a total of about 150 MR, which is enough unless you try to tank damage. Remember that your role is a splitpusher, and during teamfights, you should always try to let your teammates absorb damage and try to make focussing you hard.

But Maw of Malmortius can have its uses outside of being a third item. Even though it can delay Fiora's power spike, during the laning phase if you are against an AP champion such as Lissandra, Ryze or Kennen, building it first item can help you win the lane due to the MR shield and the MR, but it becomes more of a gamble if you cannot turn this to your advantage and win your lane, as otherwise you become less effective due to not having natural waveclear and sustain for splitpushing, you have to rely on your second item Ravenous Hydra, which gives no CDR, and you have to wait until your third item for CDR.

I hope that I have answered your question well, if you have any other questions, feel free to ask.