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).
Daggers are melee weapons that have the finesse property. The finesse property allows an attacker to use their dexterity modifier for their attack bonus and damage bonus, in place of the strength modifier.
In this case, the tiefling has a +3 proficiency bonus and a +2 dex modifier, so the attack bonus should be +5 with a finesse weapon, unless there is something missing from the sheet that explains the extra +1 to bring it to +6 such as the dagger being a magic weapon or something. That doesn't appear to be the case here because the equipment for the character is listed on the sheet but something is not adding up correctly, or else I'm missing something on the sheet completely.
As for the piercing damage question, piercing is the type of damage the dagger does. The table in the equipment chapter that details the weapons tells the reader what damage die a weapon uses and also the damage type for the weapon. A dagger does 1d4 damage, and the damage type is piercing. The damage bonus is calculated by adding the attack modifier (in this case dexterity because the weapon has the finesse property) plus any other modifiers to the damage (in this case, none), giving a total of 1d4 + 2.
I'm not certain why the sheet is showing the attack bonus as +6 to hit. All of these sheets seem to have an extra 1 that I can't figure out the source of. If someone finds it, please comment because it's really bothering me and I don't know if I'm just missing something plainly obvious.
I'm leaning towards the calculations on all of these sheets being wrong and chalking it up to an error on the first sheet not being checked and being carried over to every other sheet after. I can see no reason why the bonus is +1 higher than it should be, especially since the spell attack modifier is calculated correctly using the warlock's charisma bonus + proficiency modifier.
Best Answer
Multiple horns or no horns are technically allowed.
Tieflings in D&D 5e are described as having an unspecified number of horns, and while it is generally implied to be two horns and depicted so in art, this isn't strictly defined. It's also a valid variant to have no horns.
Multiple horns, but usually two
Player's Handbook p.42 notes:
This implies that a tiefling's horns are located at the front of their head, and that a tiefling has multiple horns, most likely two. However, it doesn't strictly say "two horns", "a pair of horns", or the like, only that they have:
It is therefore valid, by the wording of the rules, for a tiefling to have more than two horns. However, the rules do not appear to leave an opening for one-horned tieflings.
In older editions of the game, Asmodeus himself is usually depicted as having exactly two horns. This implies that tieflings should probably have two horns.
No horns
However, some tieflings have no horns. Sword Coast Adventurer's Guide p. 118, under the Tiefling Variants sidebar notes:
It is therefore possible for a tiefling to have no horns, provided they are not of the blood of Asmodeus.
Other options
You can, of course, ask your DM if you wish to play something non-standard. This kind of minor detail of physical appearance does not affect game balance.