I just use "line2" (in this particular example) because it was the second "kind" of line I encountered. Too often I encounter different pieces of poetry, in the same book, with varying indent amounts, I've become lazy now and called them simply "line2", "line3", etc.
<div> vs. <p>, I'm not going to fight about. Yes, I do think <div> is semantically more appropriate, and I could have used "div.stanza div" or similar, but I've also learnt that using classes instead of relying on a particular structure is more robust (I'm likely to find a case where I want to use a proper <p> inside a poem, or another <div> level...). As with class names, use whatever you like.
However, the margin/padding + negative indent trick is only a cludge. What we would need is:
1) Vertically centering the whole poetry block, without knowing it advance how wide it is.
2) Breaking lines by aligning the 2nd part to the right, as in the attachment. I'm not sure this is common in English, but it's certainly done in Spanish.[*]
[*] Fitting the broken part of the line in the next ("[spirital") or even previous ("[andar") line would be extra nice, but I'm not asking for that yet.