Does a durable financial power of attorney prevent the court's involvement at incapacity?
Yes, that is what it is intended to do. A durable power of attorney lets you name someone to manage your financial affairs if you are unable to do so.
A properly worded power of attorney will give enough power to your agent to do what you need them to do, but also limit your agent so he or she cannot do anything more than you want them to do.
A power of attorney will be needed for your agent to act on your behalf with various entities, including banks, mortgage companies, mutual fund companies, the IRS, the Social Security Administration, credit card companies, etc. If not drafted properly, some entities will not recognize the power of attorney and the court may end up involved. This is why you want a competent attorney drafting this important document.