If I were you, Peggy, for the NRT trip you seem ready to cancel-- I would accept the Delta Schedule change if you're rerouted onto SQ, JL or NH business class for flight times you like. They are all good airlines. My personal preference is strongly to SQ, though if you live in SIN maybe you already have an opinion about them!
Personally, if given the choice, based on lounge optioons, I'd do SQ on the way to Tokyo (Maybe not leaving so terribly early as the original DL one) and NH or JL on the return. SQ flies A380, A350, 777 on the route, while JL flies 777 and NH flies 787.

Then, after the trip, call Delta and request ORC (Original Routing Credit) for your flights and as a rule, I haven't had any issue getting miles credited for when I VDB off of a Delta flight, so in this case, you really are in an IDB/Delta's fault situation, so you should get your ORC.
That said, some posters here are saying you'll be routed onto DL's partners, meaning KE/MU/GA are the possibilities. I do know that GA business is said to be quite good, but all of those are NOT nonstop so I'd call DL and try to get them to give you nonstops.
Let us know what happens!