Mon, May 21
Tokyo --> Hakone
Cruise on Lake Ashimo, Onsen
Hakone --> Osaka
check into accommodation

That's my only concern. That's quite a bit of traveling for not much. The cruise is, what, 15 minutes long? The onsen should be relaxing, but then will you lose that relaxation by having to go from Hakone back to Osaka that night?

And JR Pass necessary if you're doing a back and forth like that? Or is it better to just pay for the higher level bullet train to save time?

Would the Kansai One Pass offer any discounts to the places you go? It's just the ICOCA card with discounts at lots of touristy places. Slight extra step to get it than any plain ICOCA card, but since you're already at the airport...