Wow, what a lovely conversation with the DSN! So glad you seem to be having some positive health care experiences at long last. Long may it continue!
I have heard mixed results, I think, about the effects of a VLC diet on carb tolerance. The standard thinking is that it increases carb tolerance because it "rests" the pancreas a bit. But - and I can't recall now where or who said this - I have a vague idea that I have heard of people tolerating carbs less well after going low-carb. Trouble is that you can't really sort out the intervening factors like time (specifically, someone whose insulin production is packing up search what they do, like a T1.5 or someone with other factors in play in their inner workings, will find that they tolerate less carbs after going on a VLC diet not because of the diet, but simply because during the time they were on the diet, they were still losing insulin production). Basically, when it comes to anything diabetes, all we can really talk about are tendencies and probabilities. The majority of people who go on a VLC diet *tend* to see more carb tolerance, but any given individual might actually see less.
When it comes to bolus insulin, insulin to carb rations (I:C ratios) are much, much more variable than what is apparently reported on Google! Just like with basal insulin, bolus insulin needs vary by person and by time of day. On a molecular level, they are the same after all! The difference is essentially in how quickly they are absorbed from under the skin. Most people find that they need different ratios for different meals, usually needing more insulin at breakfast, least at lunch, and somewhere in the middle for dinner. But again this is what people *tend* to need and your individual situation may well be different!
So for example my own ratios are 1:4 at breakfast (that's 1u of insulin per 4g of carb), 1:7 at lunch and 1:6 at dinner. So say I ate 20g of carb - if I ate it for breakfast, I would need 5u, for lunch I would need 3u and for dinner 3.5u. Obviously when you can only work with whole units, or in my case half units, the math isn't always super precise, you have to decide whether to round up or down - if i had been very active during the day I might only take 3u at supper instead of 3.5 for example, it's a judgment call!
Most people when starting out are "given" the ratio of 1:10, mainly because that makes the math easier while you're getting the hang of carb counting and everything, and also because that's a fairly "safe" starting place - it's better to start high with ratios ( = less insulin taken per meal) and gradually tighten them down ( = more insulin per meal) as you get the hang of things, rather than to start off too severe and have lots of hypos on top of everything.
As to decreasing basal - when both basal and bolus doses are titrated correctly, they shouldn't interfere with each other. Your basal dose should in principle be titrated to meet your metabolic needs between meals; it should NOT be titrated with the intention of reducing post-meal spikes. If you have been using basal to reduce post-meal spikes then yes, it will need to be "backed off" because that is what the bolus insulin will be doing too, and if you have both insulins doing the same job, the job gets done twice over which may mean a hypo for you! In theory, if you are eating such that you don't need bolus insulin to cover your food, this should not affect how much basal insulin you need on that day because your basal insulin needs (and thus your dose) should have nothing to do with what you eat or don't eat. In practice, of course, things are a bit more complicated - for example if you were not eating at all (as opposed to eating, but VLC), you could still find your BGs rising even though your basal dose works perfectly on days you do eat, because fasting causes an increase in counterregulatory hormones.
Basically, as with everything, start slow and cautious and see what happens to you, then adjust from there.