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.
What a second weapon can offer you, using only basic rules, is a second parry.
If you're highly skilled with a weapon type, let's say Rapier, then you will have a high Parry score with that weapon. Parrying twice with the same weapon is at -4(-2 for fencing weapons like the rapier) and therefore usually a terrible(fairly bad) idea, but with a second weapon you can parry a second time at no penalty*! This is useful against multiple enemies, in particular.
What All-Out Defense allows you to do, is to apply two defenses against the same attack.
In non-basic rules there are more things, like a Dual-Weapon Attack or Cross-Parrying.
It doesn't really benefit your offense, which is realistic, but it does help your defense.
*With Ambidexterity. Otherwise -2 for off-hand parrying.
Edit: After seeing your edit, I want to add that this will not cost you a lot more points. Ambidexterity is useful for all kinds of stuff, and this way you'll likely need to keep up one less skill...
Best Answer
This is true in the majority of RPGs I've seen, including all prior versions of D&D. The problem is one of level of abstraction; how specific do you really want to get with differences in defensive capabilities between different weapons? Some of the older editions (and certain versions of Traveller, for example) had varying to-hit based on your weapon and the target's armor. 4e explicitly takes a much more abstract view of combat, and parrying as something other than a reactive defense power would be inconsistent with its general view of combat.
Probably an easy fix is to assume that parrying is part of what makes up the base 10+half level to AC, and to give a bonus to-hit or combat advantage when attacking an unarmed target (because parrying weapons with your bare hands is... tricky. Doable in some cases (gauntlets, monk, etc), but hard).
I've seen a couple systems where dedicated parrying weapons (such as the main gauche and duelling cloaks) were represented as shields, providing a bonus to AC but little to no offensive capabilities. Swashbuckling Adventures did this well for 3.5 (including parrying feats), and since it's based on 7th Sea I imagine they did as well. For a 3.x variant that breaks out weapon bonuses to defense, take a look at Codex Martialis. The 40k RPG family (Dark Heresy, Deathwatch, and Rogue Trader) also feature parrying explicitly, with some weapons being well-balanced for it, and others being more difficult or impossible to parry with (chain-axes, if I recall correctly). If you're looking to create a parrying system, you could mine these for ideas.