We have similar laws in many countries including Australia. They may be ill-conceived like many pieces of legislation and it sometimes disadvantages people, but if those involved made prior arrangement, it's not a big problem and it works to protect people. DPA also protects people's information from being misused for marketing purposes etc, which is not a bad thing.

For instance I'm a nominee on my husband's BAEC account because I know more about it than he does and he wants me to manage the whole thing as he has no inclination to manage his account.

As long as access to personal data by someone else is pre-arranged, everything is good, and it does prevent stalking. Being stalked is no fun, I tell you, and if it prevents it from happening but inconveniences some who did not make prior arrangement for access to data by others, it's not a terrible thing.

Perhaps, this is another lesson we can all learn from this rather unfortunate incident that was thankfully successfully concluded thanks to the effort by the OP, some other kind-hearted people etc:

"Make sure you arrange for access to data in advance" if you are looking after a vulnerable relative. The OP wasn't to know to expect something like this so he can't possibly be held responsible for that, but we all know the potential issues now, so we have no excuse not to do so now. Let's not get the OP's aunt's suffering go to waste and we should all learn something from it.