I think you are confused...Delta is NOT buying Northwest. IF...and that is a VERY big IF, there is a transaction, it is a merger. The ONLY reason the Delta name would be kept is money. Delta lost way more money than NW during bk. Therefore they have a $9.1 BILLION NOL (Net operating loss) that can not be transferred to another carrier.
It is a stock swap.
As a matter of corporate law, the transaction under consideration may very well be a merger. As a matter of tax law, the transaction may qualify as a merger (although you appear to be confused about the NOL issues and what must (and must not) happen in order to preserve their value under the tax code). Section 368 of the Code is very complex. Suffice it to say, your take on the NOLs doesn't jive with my view.
Neither corporate law nor the tax law definitions of "merger" preclude DL from essentially "buying" NW. And neither preclude AA or any other interested airline from making Delta pay more than it had planned. A "stock swap" almost never means you get one new share of the combined entity for each share you owned before the merger.
If they're smart, the NW stockholders who are itching for a big payoff will demand that DL pay more than the current stock price - meaning that NW stockholders might get 1.25 or 1.5 or 1.75 shares of the new entity for each NW share they hold, while DL stockholders (the ones doing the "buying" of NW) will get fewer shares of the new entity for each DL share. Rarely will the acquired entity (in this case most certainly NW) simply throw their stock in the hat and take out an equivalent value of new stock. When you acquire a company, you usually have to pay a premium to current market value, and this merger will be no different. NW stockholders will be enriched to the detriment of the DL stockholders, not any different than if DL were paying cash.