I think you have understandable health anxiety due to what happened with your sister.
Re: diabetes masking pancreatic cancer etc etc.
I also think you have received good health care from your health care providers.
With the exception of the DN telling you to eat cereals after you told her you were having 'spikes' in your blood sugar. As
@Mike D says cereals are 'sugar'.
Irrespective of Diabetes, they are not advisable for anyone to eat IMHO.
However, as your health care provider and a supposed professional, I don't think your DN should have told you to continue eating them. Especially If you'd told her you were having 'spikes' in your blood sugar. As you say you took your records/readings with you and discussed this with her.
You have received very good advice from the forum users. What you should have been told by your DN is the same as everyone here has said.
If something is 'spiking' your blood sugar, stop eating it. It's common sense.
It's all a good Dr or DN would tell you to do initially, if you were diagnosed as a type 2 diabetic anyway.
Don't wait for things to progress before making healthy eating choices.
That doesn't make any sense.
You are spiking though, so you will become diabetic if you don't change your
eating habits. It's a case of watch and wait at the moment and make the changes suggested. Whether it would be Type 1 or Type 2 would only be able to be diagnosed after your situation deteriorated.
If you are concerned you may have what your sister had, then you should get a private scan done.
If for no other reason than to rest your mind.
Good luck x