I think you over thinking this. BT want all it's email users to have a MyBT BTID.
Once you have been migrated you have 12 days to decide what you are going to do with each email account. ie. keep them or gift them. I do not know what will happen with them after that. Perhaps it gets locked until you decide, I don't know.
As I have already said, the sub-account after migration remains under your control and shows as a sub-account of yours in "MyBT", "Manage BTMail" until you decide to either keep it or gift it.
The first time that the email is logged into via webmail the user , whether it is you or your partner if you have gifted it, will need to do something with it. If it is yours you will link it to your BTID via your MyBT.
if it is your partner's she/he will need to set up a new MyBT & BTID.(This makes a private email address for your partner with no access by you) This MyBT account has no access to anything that is linked to your BT email or BT account and you have no access to it.
If you want your partner's email account to be accessed by you via your BTMail, link it to your MyBT and don't set up a separate BTID for it. (In effect it is just another email address of yours but your partner can use it)
After doing the above, if you access the email via an email client you do not need to make any changes if you have already changed the server settings as outlined in this link.