[RPG] Can a 5th level fighter use a Extra Attack with a longbow

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.

Crossbow Expert

• Ignore Loading Quality on Crossbows you are proficient with (This allows for the extra attack).
• No 5ft Disadvantage
• When you use the attack action and attack with a one-handed weapon you can use a bonus action to attack with a loaded hand crossbow you are holding.

So no Heavy Crossbow 3rd attack, nor a hand-crossbow 3rd attack as a heavy crossbow is a 2-handed weapon.

Sharpshooter

• No Disadvantage at long range
• Ranged weapon attacks ignore 1/2 and 3/4 Cover
• Take a -5 penalty to attack roll for +10 damage

This is on par with each of the weapon masteries. You sacrifice accuracy (25% of the D20) for around a 50% increase on damage.

As far as the superiority dice, they're not OP, just spells for that Fighter archetype. They're on par with Paladins' Smite, Rangers' at will Hunter's Mark, and Rogues' Sneak attacks.

According the the DMG level 5-10 is when the group begins to feel its power, so it's natural for them to begin to feel more powerful. Make sure you're using the Encounter calculator on Page 82 of the DMG. This will allow for more control on your part, if you feel the group isn't being challenged enough, step up the difficulty a bit.