Basically to answer your question as understandable as possible:
Insulin forces protein and carbs in to your muscles at a super high and effect rate which in turn allows a rapid increase in weight and strength.
I have used insulin on and off for a long time and I would say 10 days of insulin taken 6-7iu morning and the same post workout will add the same as 2 months of natural training would (if I could compare to some kind of gains).
When you stop insulin I would say 30-40% is maintainable.
If you use insulin the correct way, it is very possible to put on pure muscle with no fat. Combined with t3 and some anabolics, it is possible to add lean and drop bf.
Diabetics do not take insulin to gain muscle and do not eat and train like us - if they did - yep, they would be huge if diet is spot on.
As people have said there are a number of ways to increase natural insulin levels which is safer than using insulin itself.