"My question is that can I as POA, trade in stocks which comprise his IRA portfolio, and transfer the profits to a savings account for my uncle?"
Depends on whether the company holding the IRA will recognize your POA. It isn't obligated to accept it. Besides, is your uncle incompetent? Does he *want* you to mess with his IRA? If he's incompetent, even if you have a durable POA made when he was competent, you should be talking with a family law attorney about seeking conservatorship over him and his affairs.
You might also want to contact the local agency that oversees elder abuse issues.
"... can he at this late stage of health, easily change his power of attorney to someone else such as his ' trusted cleaning lady? ' "
If he's legally competent, he's free to revoke your POA at any time or go after you for decisions you made with regard to the POA.
"Myself and my family feel that this person had preyed on my uncle at a vulnerable stage and had exploited him."
So do something about it within the existing system. :)