Hello. Firstly, let me say I am very sorry for your loss. Losing a best friend is often the hardest of things we go through, so my heart goes out to you.
I really cannot answer your questions, because canine kidney disease is extremely unpredictable, and very different for every dog. It progresses quite slowly at the start - but you may never know when it may have started, because dogs rarely show any symptoms until the kidneys are around 25% remaining in function. So, the only way is to have annual blood and urine tests undertaken of a healthy dog, and from that we have a chance of catching the disease early and making some real improvements in terms of quality of life and longevity.
The later into the disease it is diagnosed, the poorer the prognosis, even with good management and veterinary intervention.
Really there's nothing to be gained by trying to second guess the "what if" questions you have. Because, you will never know the answer - and it's impossible for anyone to tell you one way or the other. In the end, you did what you felt was the best thing at the time, and that's all anyone can ask for. Hindsight is a wonderful thing, but not helpful in this situation.
Your dog clearly enjoyed a fabulous life - and it was you that provided them with it. Try not to think of the short sad end phase, but all those many years of adventurous happy memories of times gone by.
Kind regards
Tony