Rationale for interleaving in G.709
It looks reasonable. The scheme generates four packets, each of which has a header and error-correcting information.

Since the packets have identical size, it is convenient to diagram them as rows.

The problem is your I don't know the details here. 

You may be assuming that the "2D" areas are monolithic. But perhaps they are decomposable by row. That is to say, for example, perhaps the error correction in row 1 only pertains to the payload of row 1, and isn't just one big block that applies to the payload as a whole.