[Q] Need help converting GPE lollipop to stock AT&T rom topic

Before I begin, I have already read through a lot of the guides but almost of all them are not updated enough for my case. If I am wrong please direct me to it.

I would like to restore AT&T stock rom on my phone which was previously converted to GPE Lollipop 5.1 (with root, bootloader unlocked and s-off).
Here's what I have already done.

• Restored MID and CID to 0P6B12000 and CWS__001 (for AT&T) respectively.

• Relocked the bootloader.

• Restoring from a nandroid (before going GPE) fails.

• Downloaded the most latest RUU file for AT&T model, and ran the process.

But RUU fails with error 155. And after reading online, any of the following could be cause of the problem.

1. Partition size difference in GPE and AT&T stock rom.

1. Difference in firmware version or may be HBOOT version.

1. GPE is lollipop 5.1 and I think AT&T's latest rom is 5.0.1. So may be this downgrade is causing the problem

The RUU file I was using: RUU_M8_UL_L50_SENSE60_ATT_MR_Cingula...7_signed_2.exe

I think my first step would be to flash some correct firmware to be able to successfully run RUU, but I don't know how which one. Plus, will downgrading firmware cause problems?
I would appreciate any help in resolving this problem. At minimum, I would like to be able to flash some stock rom with all the sense and AT&T stuff.