I am very sorry for your loss. If there was a Will, it should be filed within ten days of death. You will be able to view the Will at the Clerks Website for the county in which your mother passed away.
Typically Attorney's fees are handled by statute which sets a percent fee, which is 3% for estates with a value of up to One Million Dollars. Many Attorney's will also handle a probate for a flat fee as well. Typical filing fees to open up a Formal Administration with the Court is approximately $400.00.
You typically would not have to pay taxes on proceeds from a life insurance policy in the State of Florida. You should consult with an Attorney or Accountant prior to distribution to ensure the same. Further, if your Father was a resident of another State, the answer could change based on that State's laws.
You would have to hire an attorney licensed to practice law in Pennsylvania to handle an estate being administered there. You may want to post this to attorneys located within the county where you need help to find another attorney to handle the matter.
I am very sorry for your loss. If the Will was not signed prior to death, it would not be a valid devise of property. It would seem that probate will be needed to handle your father's estate. You can try to contact the DMV and see if they will allow your mother to transfer your father's vehicle to herself using his death certificate as his heir and then she can transfer the vehicle. The vehicle should be exempt from any claims of creditors.
I am very sorry for your loss. The Estate may potentially need to go through Probate in the County where your mother passed, however that will depend on many different factors, one of which being how your mother and father held the assets. You should consult with an Attorney related to what process you will have to do related to your Mother's Estate.
You can file a creditors claim for back child support. Further if the children are minors you could have an attorney Petition for a family allowance which would allow you some funds to support any minor children. Also if minors they could be entitled to additional assets as well.
Sending the Waiver prior to the funds is common practice, however I would inquire about a informal accounting, the amount of your share and when it will be sent. You typically would recieve this information along with the Waiver. As a beneficiary you are entitled to request an actual formal accounting and not at all required to sign such Waiver.
You can not really guarantee an inheritance in the future. You could buy in to the company, the person giving the business shares could change their Will and not devise you the rest of the shares. You should be careful in proceeding and ensure you properly protect your rights to future shares in the business.