GPT secondary corrupt. Is dd'ing the first 40 sectors as a backup sufficient?

I have a full disk gmirror on 12.4 using the MBR scheme.  As I'm 
migrating to 14.2S and would like to use GPT scheme, so to my questions:

1. Is there a sysctl, or flag (somewhere) to disable
" the secondary GPT table is corrupt or invalid."
notifications?  And the fear-inspiring
# gpart show md0
=>   40  32688  md0  GPT  (16M) [CORRUPT]

2. To recover a damaged primary partition, is it sufficient to restore 
from a dd of the first 40 sectors of the disk, taken after disk setup 
(formatting and labelling the disk)?

3. Is the partition header written to without my involvement?  (eg is 
the clean bit set/stored in the primary or a "sub-"partition (p*)?)

I'm sorry if my language is a bit clumsy I'm used to slices and partition.

Cheers, Dewayne
PS Sadly ZFS doesn't support labels (re MAC_[MLS,BIBA])