Hi there,
A "spike" is simply a high blood glucose reading - if you imaging drawing a graph of your blood sugar over time, if you get a high reading it'd look like a "spike" on the graph. That's what you're seeing when you get a result of 12.9 after your sandwich for example, and yes they are bad! What you want (and what non-diabetics have) is a nice even graph with no spikes.
Your doctor is right that the most important thing is your overall control (which is why HbA1C is such an important test), but the fact that you're getting these spikes indicates that you're insulin resistant and personally I would be taking steps to address this now. The doc is bang on the money that you don't want to get "sucked into the diabetes mill" - and the best way to avoid that is to take control now before things deteriorate!
You yourself say that you can see carbs spike your blood - so the prudent course of action is to reduce your consumption of these carbs! The things that'll have the largest impact are refined carbs - things like bread, pasta, rice and of course sugar. Potatoes and fruit can also have a similar effect. I'd suggest you limit or avoid these foods for a while, and replace them with better options - meat and fish and plenty of veg. Don't be afraid of eating fat - nuts and seeds, whole eggs, plenty of oil in cooking or salad dressings, avocados etc. are great sources of calories that won't spike your blood and will help with your low weight. If you want fruit, try to go for berries or melons - i.e. very watery fruit that don't contain much sugar.
Eating like this, with fewer carbs (and those you do eat coming from non-refined sources) will level out your blood reading a lot and may well reverse your insulin resistance. Seriously, you could completely avoid any further problems by addressing your diet now!
You mentioned issues with low weight, and you also need to take this into account - you need to make sure that you fuel yourself properly. Don't be afraid to eat food!!! It's no good stabilising your blood glucose if you make yourself ill in another way - but the two don't have to go hand in hand! Make sure you replace the carbs with protein and fat so you're getting plenty of calories to maintain or increase your weight depending on what you're aiming for. I can't comment on this much further, but it sounds like this is a priority for your doctor so it implies it's something you at least need to think about...? One thing to bear in mind is that being lighter isn't a way of protecting against diabetes. Type 2 diabetes is a disorder of glucose metabolism and isn't caused by obesity - it's more related to refined carb intake and how your body handles that.
As far as testing goes, you've got a few choices - either continue to test but just don't tell your doctor; stop testing for a while; or fund the HbA1C tests yourself (you can get postal tests for about £17 which aren't quite as accurate as the tests your could do, but willl give you a very good indication of whether or not it's going up.) Obviously this is entirely your choice, but if I were in your situation I think I'd improve my diet by lowering the refined carbs; make sure to increase my portion sizes and stabilise my weight using extra protein and fat; and get another test before discussing it with your doctor.
Hope that's helpful. It sounds to me like you've got every chance to take control of the situation now and avoid any future problems. Good luck, have a good read through the forums for more ideas, and do feel fre to ask any more questions!
Cheers,
Nick.