I don’t think i have the solution to your problem but. When using world partition, you only need to select 0,0 for import, and it should grab the rest for you.
However, you MUST be using world partition.
So far, I have not ben able to get anything really big imported using world partition. Only about 32x32km. Anything bigger and it seems to run out of memory.
I can however import massive landscapes using 4.27 world composition. But they wont convert over to world partition.