Yes and the executor is in charge of making sure the estate is distributed properly. If you believe the executor is not doing their job the right way you can move the court to have them removed. However that can be drawn out and costly and it is certainly better if you all could come to some kind of agreement.
Regardless you would want to present all the evidence you have. Whether or not this would persuade the trier of fact to find in your favor is impossible to tell, but based on the description it sounds like the text messages could be used as good evidence. It sounds to me like you may have a good case.
It certainly becomes harder to negotiate a settlement now that you have a judgment, but it is do-able. It would be better to have an attorney do it but if you choose, you can attempt to negotiate yourself.
You may have a cause of action against the dealer, but it will depend upon what you signed, what they said to you, what type of extra items were added (the insurance coverages you mentioned do sound pretty unusual for a vehicle purchase), and most importantly what evidence you might have that misrepresentation was involved. Because if it is just your word against theirs and you signed all the documents, it could be difficult for you to prove your case, even if you have what sounds like a good...
To start your mother should send to the Power of Attorney in writing a letter stating that she revokes his power of attorney authority. At that point he will have no authority to control her accounts and assets. Then I would suggest she contact the banks, etc to inform them that he has no authority any longer. If he still refuses to turn over what is rightfully hers, then she could start an action in court.
Look at Florida Rules of Civil Procedure 1.360. That being said, I would strongly advise against deposing someone on your own especially when the other person has an attorney. Yes you can only ask certain things, and yes his attorney can stop you from asking certain things. This is a fairly complicated thing for a non-lawyer to take on. If you are prevented from asking the right questions because the opposing counsel is constantly objecting and you don't know how to respond, it could harm...
This will vary by each municipality. Check out this page - http://www.cliftonnj.org/faqs/bfec0e44
It says that if the landlord is not complying, you should notify him/her preferably in writing before getting help from the town.