Aside from the specific changes that the Animal Companion feature makes to the wolf, it retains its statistics from the Monster Manual, so it begins with 11 hit points.

Per the class feature as described in Unearthed Arcana: The Ranger, Revised:

For each level you gain after 3rd, your animal companion gains an additional hit die and increases its hit points accordingly.

This works like any hit die. The wolf's hit die is a d8. When you level up beyond 3rd level, roll 1d8 + 1 (CON) and add it to the wolf's current hit point total to determine the new hit point total, bringing it to (3d8 + 3).

Alternatively, take the average value. Per DMG p. 276, the average for a Medium size creature is 4.5 HP per die on a d8. The wolf's new HP value is 17 (3d8 + 3).

If you increase the wolf's Constitution score when you receive an Ability Score Improvement, be sure that you update the wolf's HP total to account for the new modifier.

The formula for HP is: $$n\text{d}X + (n \times\text{CON})$$

where \$n\$ is number of hit dice and \$X\$ is the hit die value.

I have included both the option to roll for the new hit die or use the monster average because it is up to your DM to decide the method. Traditionally, taking the average is the approach used for NPCs and monsters, including animal companions.

Yes

Your companion uses your proficiency bonus rather than its own.

It is important to note that it says, without condition, that the companion uses the Ranger's proficiency bonus. This means that, from this point on, it's proficiency bonus is the Ranger's proficiency bonus. Think of it as erasing the value on the creature sheet, replacing it with the Ranger's value, and then recalculating everything from there.

Upon looking at the UA document again, this is further supported as such:

Keeping Track of Proficiency

When you gain your animal companion at 3rd level, its proficiency bonus matches yours at +2. As you gain levels and increase your proficiency bonus, remember that your companion’s proficiency bonus improves as well, and is applied to the following areas: Armor Class, skills, saving throws, attack bonus, and damage rolls.

While the wolf has a listed proficiency bonus of +2, nowhere is it stated that this bonus is already included in the damage of the wolf's attack. We may infer that this damage is from the wolf's DEX mod, as is supported by the fact that the base to hit is +4, which would be +2 from proficiency and +2 from DEX, matching expectations for standard application of proficiency bonuses. The wolf's damage should be increased by a flat 3 points, making the roll 2d4+5, the same as you have stated. Furthermore, applying +3 to the base 13 also matches your stated 16 AC, and replacing the +2 with proficiency with +3 also results in a +5 to hit when added to the DEX mod. It is not explicitly stated that the bite is a DEX-based attack, but this is the only value that makes sense from the creature stats. Your math looks right.

Bonus Answer: No, the damage dice of the wolf's bite attack do not scale. It is important to consider that the wolf companion is not intended to be as powerful as an extra PC, but is instead part of the power of the Ranger to whom it belongs. Applying proficiency bonus to a damage source is a benefit other classes do not have, and this comes from an attack that does not require the use of any of the Ranger's actions. Also, consider that this attack has the potential to knock a target prone, leaving it more vulnerable to melee attacks until it has the opportunity to stand. Such a tactical advantage is certainly not weak.