You have $300,000 at stake and you want free advice on a public forum? As knowledgeable and caring as are the volunteer attorneys of Avvo, the Q&A is ill-suited to advising in specific cases. The best we can do is provide general information about the law and legal procedure. To that end, we can say that a power of attorney is a common incapacity (not estate) planning device, and the power bestowed is as set forth in the document. Such powers are not unlimited, however. Typically the document itself requires all acts of the agent to serve the best interests of the principal and all assets applied to the needs of the principal. In many states the right of the agent to personally benefit is expressly limited. A power to make a gift of the principal's assets does not itself permit the agent to make a gift of the principal's property to him/herself. We call that theft.
Whether "his kids" or their attorney are making any of these arguments, and whether the law of your state favors their position or yours, and whether his kids can set aside your self-serving beneficiary designation are some things you will want to learn from local experienced elder law counsel. Be prepared to pay a reasonable fee.
Best wishes for an outcome you can accept, and please remember to designate a best answer.
This answer is offered as a public service for general information only and may not be relied upon as legal advice.
You need to have your own attorney look the power of attorney over and advise you. We can't do it here on Avvo. If it is worth $150,000 to you, you can afford to hire one.
As the others said, you need to hire someone; this is non-trivial. (see the answers to your previous questions on this).
You need to sit down with the documents and an attorney, and expect to pay for their review.