Larry's Pretty Good Poetry Journal

I agree, this one is thorny. I would interpret the last two lines as "Just wait until (the end of) today to see if he comes again, and then (if he doesn't) fall if fall you must." I can't point to any specific evidence in the text, but it feels more natural to me than putting the prelude to what happened all the way down in line 4, which gives you the poetic structure IMPORTANT THINGS IMPORTANT THINGS administrivia IMPORTANT THINGS. But I won't deny that weirder convolutions can be observed in other KKS poems.

I also want to interpret the "kimi" as referring to Tsurayuki's friend rather than the sakura, because that lets you read the first two lines more simply: "That person [you/we/etc] caught a glimpse of, will they come again? so thinking, o sakura, just wait.. (etc.)" (The key part is that the "ya" attaches to the "kuru" and then the whole thing gets quoted by the "to".) I'm less confident about this -- still not 100% on the rules of 2nd- and 3rd-person address in KKS times.

Hope this doesn't come off as kibitzing, btw, as I do agree that this one requires more than the usual amount of assumption-scaffolding to make sense of.