First of all thank you very much for taking time and typing a long story about your successful experience with microneedling. I was extremely happy to read about your many improvements!
Personally, I have not managed to completely erase my wrinkles with microneedling but they became significantly less indented and that makes them considerably less visible. I have observed the same with my friends who use microneedling. Their wrinkles are still there but much less indented and you have to look really close to see them. You have not used the microneedling for such a long time so it is quite probable there will be further improvement.
I see you are very dedicated to microneedling because your routine is quite time consuming. Your routine works and that is what is important. It is OK to roll /stamp with shorter needles in between the sessions with long needles.
Yes, you can replace your 1.5 mm dermastamp with your 2 mm dermastamp, especially on the areas you have described (the forehead, cheeks). However, do not use this size around the eyes or at least do not push the needles fully in. the skin around the eyes is thinner than elsewhere.
I don't think you need the single needles.
You can use your 2 mm stamp on the scar but please read my answer (#2) how to roll or needle around the eyes:
https://http://forums.owndoc.com/dermarolling-microneedling/can-i-use-a-dermaroller-on-this-area-under-the-eye-(pic-included)/There are pictures included. Do not use the 2 mm dermastamp inside the area that is circled red. There is a risk of eyeball injury, especially with such long needles. You can only needle where the bone is and not any closer to the eye. Pull the skin away from the eye when you needle/roll/stamp it:
Again, thank you for your feedback!