Can a person who is power of attorney for a diceased person, spend or distribute funds from an estate before the estate has been distributed to the other heirs?

The answer to this question is certainly "no" in Florida (and I suspect every other state).  In Florida, a power of attorney ceases to be of any use when the principal (the person who created it) dies.  The power of attorney terminates at that time.

A personal representative of an estate can spend or distribute funds from the estate.  They have to follow either the terms of the will or the state statute governing inheritance if there was no will.  However, just because the personal representative is spending money from the estate does not mean they are doing something wrong.

Personal representatives must pay the decedent's just debts from the estate assets.  They also have to pay taxes and fees.  If approved by the court, they can make distributions to some beneficiaries (for example, to immediate heirs for their living expenses).   In the end, however, the personal representative will have to present an accounting to the court that lists every asset that came into the estate and every distribution made from the estate. 

If you are a beneficiary, you are entitled to a copy of the accounting when it is prepared (usually at the end of the estate).  if you are concerned that a personal representative is misusing or misappropriating assets of the estate, you can complain to the probate court.  I suggest you consult an attorney about doing this.