At corner

At corner
 

p.134 Roland settled at his table corner [...]

The corner of a table should still be a flat surface; in this case, is at interchangeable with on?

The two prepositions are not interchangeable.
The way the sentence is written, Roland is sitting in a chair on the floor, positioned at the corner of a table.
If you change the preposition to 'on', that would put Roland on the table, at one of its corners.

Excellent answer. Thank you.