You can attack three times.
Your first ambiguity isn't really ambiguous at all. The distinction between an Attack action and an attack is pretty clear. Heck, both of the quotes in your question make that distinction.
For reference, the Attack action is defined as:
The most common action to take in combat is the Attack
action, whether you are swinging a sword, firing an
arrow from a bow, or brawling with your fists.
With this action, you make one melee or ranged
attack. See the “Making an Attack” section for the rules
that govern attacks.
Certain features, such as the Extra Attack feature
of the fighter, allow you to make more than one attack
with this action.
Just like the quotes in your question, it refers to the Attack action and an attack as two entirely separate things. So "you can use a bonus action to attack" means that you can use a bonus action to make one attack, not to take the Attack action.
Your second ambiguity is ambiguous; it could mean that each attack with a one-handed weapon allows you to use a bonus action to attack or that the Attack action as a whole does. Fortunately, it doesn't really matter, because:
You can take only one bonus action on your turn, so
you must choose which bonus action to use when you
have more than one available.
So even if every attack you make with a one-handed weapon allows you to make an attack as a bonus action, you can still only take one bonus action and make one attack.
To address your comment, we have the preceding text from the definition of bonus actions:
You can take a bonus
action only when a special ability, spell, or other feature
of the game states that you can do something as a bonus
action. You otherwise don’t have a bonus action to take.
You don't have a bonus action that Crossbow Expert allows you to make attacks with. Crossbow Expert gives you a bonus action consisting of a single attack. Even if it gives you that bonus action twice, (or up to 4 times for high level Fighters), you can still only take it once.
Yes, because it's not two-weapon fighting
The crossbow expert feat gives you the option to attack with a (loaded) hand crossbow when you attack with a one-handed weapon. It does not mention that it has to be light, so there's no restriction for that. You will also get your full dexterity modifier as bonus damage, because again, it's not two-weapon fighting.
However, the loaded part is a pretty big problem with this combo because you need a free hand to load ammunition into a ranged weapon, and a hand holding a weapon is not a free hand.
In truth, the big change over two-weapon fighting isn't that it lacks the "light" requirement, but that it lacks the "melee" requirement. The best way for this feat to work is as a crossbow-esque gunslinger, wielding a single hand crossbow and firing it multiple times in quick succession in your turn.
Best Answer
No, for at least two reasons.
Crossbow Expert (PHB, p. 165) is very specific about when you can make an attack with your hand crossbow:
You are using a reaction to make a single attack; you are not taking the Attack action.
Also, your Giant Killer feature does not apply to humans:
Humans are Medium-sized. As the name implies, you can only strike back against big creatures! (Sized Large, Huge, or Gargantuan.)