1. "Finding in these lines" means "Within the lines quoted, something is learnt" (I assume there was a quote from the poem previously)
2. Not sure what you're asking, but I'd say a simplification would be "Heidegger wants to describe a paradox, and he finds a good way to describe it within the lines of Hölderlin's poem."
3. This is a really verbose, overly-florid use of language, but I think what he means is 'humanity's restricted point of view of the world'.