Pretty sure it was in the first Novel and it happened upon their second visit to Hollow Bastion, when they went to seal the keyhole.
It is indeed presented in the "Roxas - Seven Days"-part of the first KH 2 novel, but it explicitly states it happens after SDG seal the keyhole, so it is a flashback, it doesn't actually happen after the end of KH 1.
No, it was at the beginning of the KH2 novel.
Correct, but it is a flashback to the time right after they sealed the keyhole in KH 1.
If it would have been directly after the end of KH 1 it would contradict both KH 1 and 2 because of the fact that it was stated travel between different worlds would no longer be possible once the door at the end of KH 1 is sealed.
In KH 2 Yen Sid says the same thing and reveals that "new pathways" exist which Sora can open with the Keyblade.
So the world SDG ended up in after the finale of KH 1 must have been the periphery of Land of Departure in-universe.
Meta-wise of course nothing was already decided by the time these scenes were made and the grassland and roads were just used as a general backdrop for Pluto to present that letter.