Hi imur,
For Arria 10, 0x00 to 0x1F is reserved flash header area used for image start information, not the full user image data. The actual application image must be placed at a valid flash offset. Refer Remote System Upgrade State Machine and Intel Arria 10 Remote Update Design Example
For the .rpd, I would not recommend manually patching the first 4 bytes. It is better to generate the .rpd together with the .map file, then use the .map to confirm the correct image offset in flash. Check Generating .rpd File for Intel Arria 10 and Intel Cyclone 10 GX Devices
For the repeated 0x56565656 pattern, I do not see a public document that describes the .rpd content byte-by-byte. In general, the .rpd should be treated as raw output data rather than edited manually.
Suggested next step: regenerate the file with Create Memory Map File enabled, check the image offset in the .map, and program the image at that offset.
Regards,
Fakhrul