This area of law unfortunately is clear and runs straight into the reality of family relationships and emotions. Let me illustrate. In your situation, the uncle had no right to give or sell any of the property of the estate of your grandmother. The reason I say that is apparently he was not left any property by her will. So, if that is correct, then what he did with her property is legally no different than a total stranger such as me, dealing with her property in such a fashion. The same can be said of his opinion, desire, idea about keeping the home.
Now, since the uncle is family (duh! stupid lawyer!) you have the reality of the family dynamics to deal with. Right? So, you and or your sister have to decide whether you want to just play by the letter of the law, or do as the family wishes, or some combination of the two.
The estate of your late grandmother is responsible for all debts and taxes that were left unpaid when she died. What that really means is the executor of the estate has to some how find the money to pay off those bills. Now, in California where I practice law, you can sometimes get out of having to pay off the credit card companies and others IF, the executor timely sends the creditors a notice of her death and the administration of her estate in probate. If the creditors don't respond (in CA it is in 4 months -- I think) then they are forever stopped from trying to collect on that debt. Whether or not your state has some similar program, I don't know.
The key to the above is for the executor and or the family to not make any payments to those creditors after grandmother's death, and don't make any promises or agreements with them.
I hope this has been some help to you. If it is somewhat confusing, then I would highly urge you to consult with a local probate attorney who could take this case on for you. As far as the "pay in advance" probate companies you refer to; I have no knowledge of those. There are no such animals/companies in California. Plus, in California, the probate attorney only gets paid at the end of the entire case, once it is all over. Further, what he is paid is set by law based on the dollar size of the estate.