[RPG] How to determine how many spells a character learns per level

For your specific character, you are correct.

However you left off the invocations (which can be quite invaluable and often act as a spell without needing a spell slot). I'll break this down specific to your character:

Cantrips: 2 base +3 from Pact of the Tome = 5 cantrips (+1 from Infernal Legacy covered below)

Spells: 4 known + Hellish Rebuke - these spells must be selected from the Warlock spell list. The Great Old one patron allows you to select from an expanded spell list which includes the spells listed under Great Old One on PHB pg. 110. This means you can select from these spells when you learn a new spell, not that you know them in addition to the ones you do select.

Invocations: 2 invocations, these do not count against spells known. There is no requirement to know the spell in advance, otherwise this would be addressed under prerequisites in the invocation. In fact, the Invocations have a different name, and you can infer that the name of the Invocation is the Warlock version of said spell, with the spell name under said Invocation being used as reference to it's capability. This is demonstrated through Invocations such as Armor of Shadows which is effectively Mage Armor without the spell slot or material component requirement. Another example is Book of Ancient Secrets, which besides letting you pick any 2 rituals from any class (wow!), it allows you to cast a lot of your Warlock spells as rituals so you don't have to expend a spell slot (which are invaluable as a Warlock).

Infernal Legacy: All spells known from this are separate and distinct from your spell slots and invocations. These do not use spell slots to cast but function on a specific recharge as indicated by the description under Infernal Legacy. This is separate and distinct because it is not a Warlock feature, but a Race feature and does not require spellcasting or pact magic in order to utilize (it's in effect the same thing as a Dragonborn breath weapon).

Can I use bonus slots to cast spells above what my class level normally allows?

No.

According to the official Paizo FAQ, you can only use bonus slots for spell levels that your class and level has access to.

For an example of this in action, the Paladin has a "0" in his Level 1 spells per day entry at level 4 (note that up to level 3, he has a "--"). This means he is granted 0 level 1 spells but can use his bonus slots, if he has any, to cast level 1 spells. As far as I am aware, only the ranger and paladin classes fall under this scenario, where bonus slots might grant casting before a class feature gives any per day spell slots.

Copying A Scroll into a Spellbook

There are no restrictions on spell level aside from the usual checks required to copy a spell into the spellbook. The wizard deciphers the magical writing on the scroll and once he has done so, he studies the spell for a minimum of one hour. At the end of the hour he makes a spellcraft check (DC 15 + spell level). If the check is successful, the spell is copied into the book regardless of the level. Copying the spell into the spellbook also costs GP based on the spell level; the exact costs are found in the table under the header Writing a New Spell into a Spellbook on the PRD page for Magic. If the spellcraft check fails he can't try again until a week has passed; the scroll is not consumed.