How would I answer a question on love/music in this unseen passage from The Mill on the Floss?

Great question. I've had a look over the passage and decided to focus on the extract below to give you an idea of how I'd go about approaching a close analysis. I think the key here is to 1) underline any evidence you can use to support your argument before you start writing, and 2) make sure you show how the two concepts relate to one another.
[Philip]: ‘Ah! perchè non posso odiarti.’ I don’t know the opera, but it appears the tenor is telling the heroine that he shall always love her though she may forsake him. You’ve heard me sing it to the English words, ‘I love thee still'. [...] Stephen rolled out, with saucy energy,— “Shall I, wasting in despair, die because a woman’s fair?”, and seemed to make all the air in the room alive with a new influence.
You can see a difference in Eliot's diction describing the music sung/quoted by the two men here. The ‘type’ of love conveyed by Stephen is one rooted in infatuation: the heady, overwhelming sort of passion that has the impetus to command a room. Everything from Stephen’s own “saucy energy” to the content of the song itself – describing a love so ferocious in its intensity the individual is dying in its absence – communicates his feeling for Maggie without explicitly stating it. Just as music performed a materialising role in Maggie’s adolescence (allowing her to express, through metaphor, an intangible feeling) it adopts a similar function in her adult life, utilised by the men to convey emotion implicitly. The force of Stephen’s infatuation is exaggerated further when juxtaposed with Philip’s choice of music: an operatic piece by Bellini, La Sonnambula. Unlike Stephen’s choice of song, Philip’s decision is steeped in the traditionally romantic; it is centred around sacrifice rather than frustrated passion. La Sonnambula resonates with his personal circumstances – that of his relationship with Maggie, founded on their shared childhood. Sentiment prevails over physical attraction, on account of Philip’s deformity, and therefore Philip’s ‘type’ of love is the enduring, familiar kind existing in stark contrast to the flash of emotion presented by Stephen.