As I have indicated, I am deleting everything in the disk as I convert to GPT with same size MD partition. I have about 32G of free space to work with as the current MBR partition 1 is empty/unused. So, there should be no issues with secondary table at the end. The new partitions will automatically move up a bit and RAID rebuild will take care of initializing the new partition after I clobber the disk and recreate a new GPT layout.Yes what you've described will work except for having to shrink the second partition a little for the backup GPT header. mdadm guides have got you there; should be pretty simple for a RAID-1 that's not totally full.
I am not too worried about any redundancy on EFI/boot partitions as a broken EFI is easy to fix with booting install disk in rescue mode and reinstalling grup again.Most people seem to opt for having two ESPs that are kept in sync and thus both/either are selectable from the firmware boot process.
Regards Ramesh