1) Why isn't it always getting an adamant nature?
It is only a 50/50 chance that the parent holding the Everstone will pass it's nature down. (Fun Fact: In B2W2, it a 100% chance,)
2) Why isn't it getting perfect IVs?
Ok, rewind. You will get 3 IVs from the parents meaning 1 IV from let's say. Ttar, and 2 from Ditto, not 3 each (or the other way around). They will also be different (Both parents will not give their Atk IVs.) If you give Tyranitar the Power Bracer, it will pass down it's Attack IV and then 2 different IVs are passed down from either parent. If both hold Power Items, the baby will get one or the other, not both.
(Ditto + Power Weight) + (Tyranitar + Power Bracer) = Baby with either Ditto's HP IV OR Tyranitar's Atk IV. Then 2 different random IVs other than Attack or HP, depending on what you get.
The last 3 IVs are random.
3) If the Larvitar DOES have perfect IVs, what will it's characteristic be?
(Often dozes off, Likes to thrash about, Capable of taking hits, Mischievous, Somewhat vain, Alert to sounds).
It does not matter which one you get. The game just picks a random IV of 31 out of the perfect group and gives you that characteristic.
HP: | Attack: | Defense: | Sp. Attack: | Sp. Defense: | Speed: