The key question for me is whether I have a duty to manage my mothers assets actively to maximise her income, when truth be told if everything were sold off, she could probably afford to spend the rest of her life living in a suite in the Savoy or on a P&O cruise.
No, you don't. An attorney has the same responsibility to the beneficiaries of the donor's estate as the donor would in the absence of the power of attorney: none. Beneficiaries have no grounds for complaint if upon the death of the testator the cupboard is bare, whoever emptied it.
The Power of Attorney dies with the donor, as do the responsibility. As has been discussed either here or on MSE (I forget), investigations by the OPG into misuse of powers of attorney die with the donor as well: they are not interested in mis-use unless there is a real live donor, rather than the donor's estate, who would stand to benefit from any recovery.
You can argue this gives attorneys a lot of latitude and the donor rather less protection, but there it is. Dishonest attorneys have very little to fear, provided they keep the donor's affairs ticking over such that the donor is provided for until they die.
In other words, if the attorney were to manage the donor's affairs such that the donor is comfortably provided for in their accustomed style, but the attorney was also lining their own pockets such that the day the donor dies the accounts simultaneously hit zero, they are practically in the clear. Although it is unethical, immoral and on paper illegal, in practice no-one has the locus to bring a successful action. The OPG won't (can't?), a criminal action for financial abuse would almost certainly fail and the beneficiaries have no standing. It's theoretically possible an action could be brought, but the hurdles to success would be extremely high. If anyone has case law, it would be very interesting to see it.
On the other hand, if the attorney emptied the donor's accounts ten minutes after the donor's death, that's flatly illegal: the POA and all its powers died with the donor; even if the attorney is also an executor. After the donor dies they have only the powers (and responsibilities) of an executor, at best.
An attorney has no responsibility to the beneficiaries, and no responsibility to the donor other than to manage their estate with reasonable diligence, which is a much lower standard than "as though they were a regulated financial advisor". If someone wants to make a regulated professional their attorney, and pay for it, that is their choice and then (I assume) the actions of the professional would be held to an appropriate standard. But it would be clearly invidious, and unreasonable, to expect a random holder of a POA (usually the donor's child, usually these days approaching if not already at retirement age) to transform themselves into a financial advisor. And that is not expected.
All that said, holding a power of attorney is doing someone a massive favour. I wouldn't be the POA for someone who had played silly buggers with their will, and I suspect I would not accept POA for someone whose will I didn't know the contents of. Being old is not an excuse for being spiteful, and I would place my own and my children's financial interests ahead of my parents'; if there was the slightest suggestion that operating a POA for my parents would cause problems for me because of decisions that my parents had taken, I would chuck the whole business to the state and leave them to it.
It would be outing to give details, but I have cousins whose mother has absolutely shafted them with a POA which has limitations which make it effectively impossible to effectively manage her affairs. It wasn't done out of spite, but it was done out of a combination of ignorance and stupidity. I have a lot of sympathy with them now, but had I been presented with that POA I would have refused to accept it and let the dice fall where they might. They knew its contents while she still had capacity, so could have told her to fix it or find other attorneys. They didn't ("you know how she is" and all that) and are now in a complete mess.