I can't say I've ever found a swamp completely surrounded by plains although swamps do often seem to share a border with them. As for the lack of burial chambers, it seems to me that they're scarcer than they used to be. But it sounds like you have a rotten seed.

The best place to build a base in the swamp is in the trees. The big trees there are indestructible. Not even Bonemass can hurt them. Build stairs up 20m or so and build there. All pieces anchored to a tree will be blue so using several trees can get you a large tree house where only the wraiths can reach you.