Flesh // Blood in your hand has two values for converted mana cost: 5 and 2. As one of those CMCs fits Kari Zev's Expertise requirement, you can cast the Flesh // Blood card.
When you do cast it, you can cast either side (regardless of which CMC that side will have on the stack), and as you're casting it from hand you can even Fuse it to cast both sides.
As the rule you quoted on the second post says, when the fused Flesh // Blood resolves, you follow Flesh's instructions first, and then Blood's.
As of Amonkhet, the answer to comment 1 has changed.
Now, as of Amonkhet, "[t]he mana cost of a split card is the combined mana costs of its two halves" (C.R. 708.4b). Moreover, Kari Zev's Expertise says "cast a card ... from your hand", not "cast a spell ... from your hand", and that "card" refers to the card in "your hand" (compare C.R. 109.2a with C.R. 109.2b, respectively), so what's relevant here is the converted mana cost of the card in "your hand" (not the converted mana cost of the spell it will become), which will be the total converted mana cost of both halves (C.R. 202.3) what characteristics the corresponding spell has is irrelevant. All this is the case regardless of whether the card has fuse. See also this thread.
For the purposes of Kari Zev's Expertise, what is relevant is what characteristics the corresponding spell has on the stack.
Thus, for example, Flesh & Blood now has a converted mana cost of 7, not 5 and not 2, while it's in a player's hand, so it can't be cast with Kari Zev's Expertise since its converted mana cost is not 2 or less.
EDIT (May 27, 2019): Correctness edit.
EDIT (Oct. 1, 2019): See comment 6.
EDIT (May 5, 2020): Edited to conform to update with Ikoria: Lair of Behemoths.
As of Throne of Eldraine, the answer to comment 1 has changed again.
When that set is released (and in prereleases):
For the purposes of determining whether you can cast a split card from your hand with Kari Zev's Expertise, what is relevant are the characteristics of the half you choose (or, if the card has fuse and you so choose, the characteristics of both halves) in your hand before you begin to cast that card (new C.R. 601.3e; C.R. 708.3a, 702.101a).
With Kari Zev's Expertise, you can, for example, cast from your hand the Blood half of Flesh & Blood, since its converted mana cost is 2, but not the Flesh half, since its converted mana cost is 5, and not both halves, since their combined converted mana cost is 7 (C.R. 202.3, 708.4, 708.4b).
EDIT (May 5): As of Ikoria: Lair of Behemoths, the answer to comment 1 has changed once again, but not in a manner that seriously affects this answer in general. Now, for the purposes of determining whether you can cast a split card spell from your hand with Kari Zev's Expertise, what is relevant are the characteristics of that spell. This will in general be the characteristics of the half you choose (or, if the card has fuse and you so choose, the characteristics of both halves) (C.R. 708.3a, 702.101a).
EDIT (Feb. 28, 2021): Struck out certain text for correctness.
4/15/2013 When a fused split spell resolves, follow the instructions of the left half first, then the instructions on the right half.
Here's the deck idea https://www.mtggoldfish.com/deck/588988#paper
When you do cast it, you can cast either side (regardless of which CMC that side will have on the stack), and as you're casting it from hand you can even Fuse it to cast both sides.
As the rule you quoted on the second post says, when the fused Flesh // Blood resolves, you follow Flesh's instructions first, and then Blood's.
As of Amonkhet, the answer to comment 1 has changed.Now, as of Amonkhet,"[t]he mana cost of a split card is the combined mana costs of its two halves" (C.R. 708.4b).Moreover, Kari Zev's Expertise says "cast a card ... from your hand", not "cast a spell ... from your hand", and that "card" refers to the card in "your hand" (compare C.R. 109.2a with C.R. 109.2b, respectively), so what's relevant here is the converted mana cost of the card in "your hand" (not the converted mana cost of the spell it will become), which will be the total converted mana cost of both halves (C.R. 202.3) what characteristics the corresponding spell has is irrelevant. All this is the case regardless of whether the card has fuse. See also this thread.For the purposes of Kari Zev's Expertise, what is relevant is what characteristics the corresponding spell has on the stack.
Thus, for example, Flesh & Blood now has a converted mana cost of 7, not 5 and not 2, while it's in a player's hand, so it can't be cast with Kari Zev's Expertise since its converted mana cost is not 2 or less.EDIT (May 27, 2019): Correctness edit.
EDIT (Oct. 1, 2019): See comment 6.
EDIT (May 5, 2020): Edited to conform to update with Ikoria: Lair of Behemoths.
When that set is released (and in prereleases):
For the purposes of determining whether you can cast a split card from your hand with Kari Zev's Expertise, what is relevant are the characteristics of the half you choose (or, if the card has fuse and you so choose, the characteristics of both halves) in your hand before you begin to cast that card (new C.R. 601.3e; C.R. 708.3a, 702.101a)., for example,cast from your hand the Blood half of Flesh & Blood, since its converted mana cost is 2, but not the Flesh half, since its converted mana cost is 5, and not both halves, since their combined converted mana cost is 7 (C.R. 202.3, 708.4, 708.4b).EDIT (May 5): As of Ikoria: Lair of Behemoths, the answer to comment 1 has changed once again, but not in a manner that seriously affects this answer in general. Now, for the purposes of determining whether you can cast a split card spell from your hand with Kari Zev's Expertise, what is relevant are the characteristics of that spell. This will in general be the characteristics of the half you choose (or, if the card has fuse and you so choose, the characteristics of both halves) (C.R. 708.3a, 702.101a).
EDIT (Feb. 28, 2021): Struck out certain text for correctness.