I don't know why no one is answering these questions...
Here's my 2 cents.
I think the x-ray would be fine.
I would completely avoid all flouride treatments now and in the future - I don't think they will do you any good and flouride is bad for your thyroid.
I would use the novocaine and then avoid nursing until the novocaine has completely worn off by you. IMO, this means your body has processed the novocaine.
Do you have a biologic dentist? If so, you can ask for a numbing agent that is nursing-friendly. There are several kinds of anesthetics.
Also, avoid any silver fillings as a replacement!
In preparation prior to the appt, if you have the ability to, pump and store a few bottles for your NB and then pump & dump after your appt.
If the filling is silver that is being taken out, there are more safety concerns and a longer window to avoid nursing.
This is probably too late, but I didn't want your question to go unanswered.