So Tolkien agreed that the story needed to be aggressively rewritten to match, and that didn't seem to him to be disrespectful to the original book.

If Tolkien had indeed aggressively re-written The Hobbit to bring it into conformity with The Lord of the Rings (as he began to do in 1960), I would judge that to be very disrespectful to the original novel indeed (assuming said re-writing also entailed suppression of the original, a la George Lucas). As the situation currently stands, Tolkien's partial rewrite in the second and third editions can be forgiven on the basis that the original text is still accessible, and he even found a way to incorporate both the original and revised versions into the Middle Earth "canon" (with the 1937 edition reflecting the events as told by Bilbo and the 1951/1966 editions reflecting the events as they "actually happened").

Furthermore, surely there is something of a difference between an artist aggressively adjusting his own work and a third-party adapter doing the same thing?

Bit of trivia: Gandalf can't remember the names of the two blue wizards in the film. This is likely because their names were not completely consistent, and the only sources for their names are from incomplete stories. It was a joke based on the two different versions of their names.

Interesting. I had assumed Gandalf's memory loss had something to do with Jackson & Co. not owning the rights to the names Pallando and Alatar (they only have the rights to The Hobbit and The Lord of the Rings, including the Appendices; other of Tolkien's works on Middle Earth, like The Silmarillion and Unfinished Tales, are off limits).