Don't go talk to a lawyer, hire one asap. Do not make a single move without the assistance of a lawyer. Do not talk to anyone about anything related to this situation. Refer everything to your lawyer. Stop posting on the internet. It is discoverable (i.e. you may have to produce copies to the adverse parties in litigation).
I hope your grandmother did the second will cutting everyone out with a competent lawyer. If she did, it will make a huge difference in a will contest in probate court. There are a number of bells and whistles that can be used in a will to help prove it up in court and beat back the disgruntled relatives who got cut out. If she did not, you could be in for a long fight, not because the relatives have meritorious claims, but because probate court is slow and everyone is entitled to an appeal if they lose.
I would also be curious as to what kind of estate planning went on. I really doubt your grandmother would just turn loose 10.5 mil in cash and subject @4 mil to significant estate taxes. There should be trusts set up in the will to avoid estate taxes. It is estate planning 101 for lawyers and tax professionals.
While it sounds like you may have a very solid shot at the 10.5 mil (largely dependent on the quality of the legal work drafting the original and second will), you may want to bite the bullet and pay off the uncle sooner than later. You might ask your lawyer to push to get everyone to mediation very early on to see whether you can all reach an agreement. Otherwise, there will be significant legal fees (your attorney will probably work on a mix of hourly and contingence fee (translation: six figures in attorney's fees at the least if there is a will contest) and a long time (potentially years) before you finally prevail and get the money. Also, there will be depositions, hearings and a trial if everyone fights it out. It is fun to watch other people go through that, but not fun at all when you are in the hot seat.
But also be prepared for the disgruntled relatives to be irrational and greedy. Will contests always bring out the worst in people. That is why you need a lawyer (a good one who litigates in probate, not your college drinking buddy who just graduated from law school and has opened an office in his mother's garage).