[Coldsaturn]

The way I've always interpreted the last lines was that it's warning against using the fountain of atei if you're not already dying.

"when flowers wither in their sleep, when winds fall silent, when the vigilant owl closes its eyes -- only then can you drink deep of these waters"

I too tried to see if Lost Lover and Young Traveler could refer to the LIs or one LI and MC, but it wouldn't make much sense to single them out in Caleb's branch. It could mean "you, generically young and healthy person, stay away from this" and "you, heartbroken person, go with alcohol instead of this maybe"?
But yeah, how exactly the fountain needs to be used is something very relevant to the main story (especially the new section Death&Rebirth), and the consequences of misuse might just have to do with all the body corruption going on.