Defending others works the way the first paragraph you quoted says so:
You can even make defend actions on behalf of others, so long as you fulfill two conditions: it has to be reasonable for you to interpose yourself between the attack and its target, and you have to suffer the effects of any failed rolls.
That's it! That's defending others. If you can justify doing so, you can get in the way of the attack, and defend against it yourself, and you take the consequences of doing so.
The extra stuff confusing you isn't the rules for defending others. If you want to defend someone else, this paragraph is all you need to keep in mind.
The section from FC160 is just saying you can take a hit if you want.
FC160 only points out that when you're defending, if you want to, for whatever reason, you can simply not roll at all and leave your defence at +0. That includes defending yourself, or defending others.
It gives an example of doing this when defending someone else from an arrow, but that's just an example. You could also roll to defend against that arrow.
Sometimes, though, you won't be able to. If you really want to save that person from the arrow flying at them, it might be that your only option is to take the hit yourself. Most people can't deflect or stop arrows, and you might not have a shield handy.
The defending stunts you quoted are from the Fate System Toolkit
The Fate System Toolkit isn't there to give you Fate Core/Accelerated material, it's there to help you mess with the Fate system and reconfigure it. The material in that book is there to explore mechanical possibilities, not suggest highly effective character options. Not all options will be appealing under Fate Core's configuration of the Fate engine.
The two example stunts you saw were for that exploration: one was an example of a particular configuration option, and the other was just a stunt on an example character sheet. (Example character sheets don't have to be mechanically awesome!)
Plus, not all stunts are inherently and always useful for everybody. They don't have to be. Many character options will range from useless to stellar depending on your story's setting and tone, the capabilities and power levels of your characters, the kinds of challenges you'll face, and other factors. Different Fate implementations also value stunts differently — while Fate Core allows stunts which grant +2 to something to always apply, Masters of Umdaar limits +2 bonuses to once-per-scene activation because of its different focus.
One of those stunts, though — Dwarven Shield Maiden — looks pretty good to me. A Dwarven Shield Maiden's aspects probably give her a lot more justification for being able to defend against an attack even when others wouldn't be able to. Can you leap in front of an ally and defend them from a hail of bullets at +1? Probably not, but a Dwarven Shield Maiden can.
Defend the Weak could also be okay. Because it operates off of Fate points, it may even leave you able to defend when you otherwise couldn't, e.g. you're bound up and immobile. Doing so may even be a self-compel. This is something that'll vary, as above, depending on the context it's being used in.
You are almost exactly correct.
All the math you have is accurate. However, you've missed a subtle rule mentioned in the PHB (pg. 202) regarding spells cast as Bonus Actions. Namely, when casting a spell as a bonus action
You can't cast another spell during the same turn, except for a cantrip with a casting time of 1 action.
Thus, while it is possible to reach 27 AC using the methods you've described, you cannot cast Haste and Shield of Faith on a single turn.
A rule in the book for this case is the Advantage rule.
With a character making little to no effort to defend themselves, you create a circumstance where the attacker has advantage.
Advantage means two d20 rolls for the attacker; use the best score.
Simply advising the DM that you are doing nothing to defend yourself may cause the DM to give the attacker advantage. While there isn't an action called "anti dodge" not defending yourself is the opposite of the dodge1 action, which imposes disadvantage on the attack (mechanically). You can set up a signal with your DM -- "I am going to anti-dodge this round" -- to help speed up play. (Work with your DM to fit this to your table).
A simple way to create advantage for an attack upon you: Fall Down
If there is a melee attack being made upon you, falling down creates the prone condition. Your character being prone will provide the attacker with advantage on the attack (Appendix A; Conditions; Prone) within 5' of the prone character. (An exception to this is a lance attack which has disadvantage on attacks within 5')
The attacking enemy can still screw up on a given attack, in a general sense; rolling a 1 for a melee or ranged attack is always a miss2. Even in a case where the roll is made with advantage, which depending on AC will accrue a to-hit bonus of +4 or +5 (in terms of probability), the attacker can screw up and not make an effective hit.
A complete miss is less likely with advantage on an attack, but it is still possible.
Become paralyzed(the character who wants to get hit).
The character can apply some Crawler Mucus in order to get paralyzed.
Another solution is to not roll the dice
See Tom's answer for why rolling the dice need not be necessary. Discuss that with your DM.
About that shield(if being wielded)
There are not infinite spell slots. Based on the referenced GiTP thread, and presuming not-a-five-minute-adventuring day: if the character inviting this attack has a shield, but wants to be hit to exploit Armor of Agathys, then they are better off using their action that turn to doff (remove) the shield (it takes on action to do so) which removes its +2 from the armor class.
The dexterity element of armor class isn't as easily discarded, but with the benefit from advantage being about +4 to +5, the advantage should at least take care of the dexterity bonus to armor class. (Depending upon the attackee's dexterity score).