I have to take issue with #2. Whilst the poem is overtly about Tennyson's struggle to come to terms with the death of a deeply-loved friend, this is used to express Tennyson's reflection on the conflict between religion and belief in God and science, which seemed at the time to disprove God and religion. Let us remember that Tennyson wrote this poem at a time when new scientific discoveries such as Darwin's Theory of Evolution threatened to do away with the need for God and challenged widespread views regarding issues such as the age of earth. This is why the poem makes reference to faith and belief in God, and also to "nature red in tooth and claw."
The poem is a tribute to a lost friend, but the friend's death makes Tennyson stop to think about life and faith. A loss such as this one often makes a person reconsider elements of faith, and when faith is shaken it adds to the already difficult time of losing a friend.