I'd be surprised if they're writing down in-service assets, but maybe? For sure the $400 million charge includes whatever was on the balance sheet for the 4 impaired assets now sitting in the desert. The 744 refurbs were a fumble by DL, they're smart to take advantage of the current climate to sweep it under the rug.
Anderson did some CYA on the 744 refurbs. The decision and risks were presented to the BOD, who ultimately voted on the issue.
IIRC, NW was unable to change the terms of any leased planes when they went into BK. I'm sure there are a ton of factors that play out, but the main one I think is they want to fly smaller loads directly to the TPAC destinations.