I can't see a thing wrong with any of those results. They are all absolutely normal.
If you were already low carbing before your oral glucose tolerance test, this will have skewed the results. This test requires normal levels of carbs for 2 or 3 days prior to the test for it to work properly. I can sort of explain why if you want me to. Even then you only failed by 0.3mmol/l which is insignificant because of meter inaccuracies, and looked at alongside your HbA1c (which was excellent) I would say you are worrying over nothing, particularly in view of your own finger prick tests.
If I were you I would carry on as you are, have an occasional indulgence (maybe that small piece of cake once a week?) but keep testing to make sure you don't start to trend upwards.
The dietician was scaremongering. There are many T2s on here with much worse levels than yours but not on medication, and many others who started on meds, including insulin, that have managed to come off them all.
To answer your question, I wouldn't be worried.