Hi Binky! The answer to these sorts of questions is always a bit of both. That is, there are things you can do, or find out, and things that, in practice, you can't. Going point by point from your post:
Pinpointing when your endogenous insulin kicks in and by how much is one of the things that you can't really do. In general, the pancreas starts producing insulin almost from the moment you start chewing. However, in T2, a lot of times (but not in every case) what's called the first-phase response is broken or impaired. So the pancreas isn't on board until a bit later in the digestive process. Given all the other factors that go into how much BG rises after eating and when, such as the ones you mention, and given that you have injected insulin and gliclazide as well, it's going to be pretty much impossible to pin down. And even if you could, it likely wouldn't be at exactly the same time every time. These are quite literally organic processes so they tend not to run like clockwork!
As to your question about timing, there are a couple of different points I'd like to make. First, 2-2.5 hours is not an ideal post-meal test time to spot your peak BG; it's generally going to be too late, at that stage you will have already peaked (in theory) and be coming back down. The best thing is to test intensively after meals, every 15-30 minutes or so, until you find where your own personal peak is. Obviously that uses a lot of strips. As a more conservative sort of half-measure, assume your peak is somewhere between 60-90 minutes after the first bite and test then.
Second, if you test your BG post-meal and it's gone up by 2-3 points, it's not that the insulin isn't doing anything, in fact it's working just as it should. It's actually working very hard in that first hour or so after the meal and you would have gone up *much* higher without it. We just have to manage our expectations about what insulin can really do post-meal. It likely won't be able to keep us completely flat or nearly so. A rise of 2-3 points post-meal is perfectly within target. As a T1 I often have to be content with an even bigger rise. As a T2 you may be able to work out a good combination of diet, exercise, and meds that will help control it more closely.
Third, some of the problem might be that you are estimating carbs roughly. Which is not to say that you need to count carbs to every 0.5 of a gram, especially if you're not precision-titrating your insulin dose to suit, either. But an experienced carb-counter will tell you that it is VERY easy, especially when you're not yet used to counting carbs, to mis-estimate carbs. Hidden carbs especially can wreak havoc (that is, carbs in foods that you wouldn't expect to have carbs, like tomatoes and onions, soups and sauces, etc.) Part of the problem may be that at some meals, your estimate is more accurate than at others.
Fourth, as you may expect, this is one of those things that both is and isn't controllable. Having your BG not respond the way you expected to insulin and food is pretty much the name of the game in diabetes. I mean, if it always went according to plan, life with diabetes would be fairly straightforward and we wouldn't need forums like this very much!
On the other hand, yes, it is something that you can work at and in getting to know your own body's patterns, come up with some solutions that get you more like the results you want (for example, maybe for certain meals you need to take your insulin a bit ahead of when you start eating, not right as...and maybe for other meals you don't). Just know that nothing is ever set in stone, and even after 20+ years with diabetes, your body can throw you a curveball! So it's really just about doing the best you can on the one hand, while accepting on the other hand that you will never achieve total perfection with it. More important than perfection is persistence - to keep working at it consistently for the rest of your life without burning out!
About the changes in insulin dose: Yes, to some extent, changes like this are to be expected. It's early days for you in being this involved with your dosing, so you don't really have the background knowledge of how your body tends to behave to know if this is normal or unusual just yet. So you kind of just have to keep an eye on it and wait and see. But insulin needs do fluctuate all on their own, and they can do so by quite a bit. Sometimes there are phases where it feels like you might as well be injecting water for all the good it seems to do! Remember that hormones (including stress hormones and female hormones) and even colder weather can all play a part. Whatever is having an effect on your insulin needs would be having the same effect on your naturally produced insulin as well, so it makes sense that the gliclazide would be having less effect (or rather, your endogenous insulin that the gliclazide is making you produce is having less effect).
Of course, and not to worry you, but it's within the realm of possibility that your pancreas may be exhausting itself. One of the worries with gliclazide is that since it does essentially force your pancreas to do overtime, it can speed the way to the "beta cell burnout" that some T2s experience, leading them to produce very little endogenous insulin at all. On the other hand, injected insulin can do the opposite, and give your pancreas a bit of a rest, as you've suggested. At this stage, though, it's impossible to know. All you can do is wait and see, and keep trying to keep up as best you can with your BGs via injected insulin. If things settle back down in a few weeks, you'll know it was just one of those phases we all go through. If not, you may want to see what your GP has to say about it. You may want to do that anyway, if you're concerned!
Finally, about the only thing you can really do to match insulin to peak BG is to change when you take your insulin relative to when you eat. But even then it's very difficult to get it just perfect. Matching insulin to food, in terms of dose and timing, is one of the two key battles of any insulin user (the other is getting the basal insulin right, but you're not taking any at the moment). So nothing surprising about the fact that you're finding it difficult - again, if this kind of thing would just slot into place with a few minor tweaks, us 20+ year old-timers wouldn't be on forums like this! Every meal peaks at a different time, although usually you can expect it somewhere between 1 and 2 hours post-meal. But the meal may continue to digest for quite a while after that, or reach the system very quickly, or... And on the flip side, rapid-acting insulins reach peak action period around 2 hours post injection, and start tailing off after that. But exactly when depends on the type, the size of the dose (the bigger the dose, the slower it will be to peak - rule of thumb is to never inject more than 7u in one go, so if you need say 10u at one meal, take 2 injections of 5u each in different spots), the injection site (microscopic, uncontrollable differences in the skin, blood flow, etc. that can make some injections absorb better than others), and the person (since everyone is different, after all!).