My deepest sympathies, Its tough to have to do something like this at a time hen you are grieving.
My advice is to treat this like a Project and you have an awkard customer.
Your responsility as an executor is to execute the will. You do not have a responsibility to look after the beneficiaries.
Write down the plan in detail to sell everything and divide the proceeds as per the will.
and inform everyone by letter of this plan,
Get legal advice written down.
Keep everyone informed regularly of progress.
If there is a reason why things are blocked because a beneficiary will not respond, get legal advice on what to do, when write to everyone and tell them what you are doing.
If she ignores the correspondence, ignore her and move on.
Ignore the van. She is trying to pick a fight, don't let her.
Simply inform her that it needs to be taxed and insured on xxx date and the estate needs to put the tax/ insurance disc on the car.
Write and tell everyone that you paid the funeral expenses out of the estate, as you had to as the executor.
Be formal and polite.
You are doing the right thing.
Good luck.