In case you know this please forgive me as others are surely in similar circumstances and may benefit from your situation.
Cerebral atrophy means that your brain has shrunken in size and is usually having minimal function. I am not sure if mini strokes you mention are Ischemia (normal blood flow is interrupted for short periods of time causing losts of very small areas of damage) or small areas of brain death (strokes that affect small areas). Either way, you have brain damage.
Dementia happens when the functions of the brain are changed due to changes in the brain structure. What you experience is nerve messages not getting where they need to go because the road is blocked or entirely gone. This can make it hard to do different things based upon the parts of the brain that have been damaged. Some people can't speak well - others can't remember well -- some can't coordinate movements well. Some have severe problems others have lots of less severe problems.
Aricept is supposed to help if there are plaque blockages and some nerve signals can still get through. It has side effects.
The smoking made your condition much worse. Episodes of high blood pressure alone could also cause this damage. Diabetes definately makes your whole body worse. By not smoking and controlling diabetes and blood pressure, you will help yourself a great deal.
Assuming that your allergy was causing you to die by stopping your breathing and dropping your blood pressure - they had to do something - but if the allergy was treated they probably should have stopped when you got the heart symptoms.
There is nothing you can do about the past, sir, but you can try to make sure that medical professionals are being trained better these days by advocating for patient case reviews by independent boards.
For yourself - I would guess that most of your fatigue and bad feelings probably came from the trying to cope with your condition and that the diabetes and increasing brain damage made it worse.
Work to keep your diabetes controlled. Make sure you have a great support network to help you with your memory and other problems and plan to do less mentally straining work -- like work that involves routines which your brain may still be able to handle. Use the pathways that still work as well as you can. Know your limitations - for example if you get lost, do not drive a car or do not travel without a companion.
Make sure the aricept is helping and useful. Make sure you keep your blood pressure under control - you have a reactive blood pressure but I am sure that stress is also making it high. If working is stressful, you may have to go on a disability program.
If your country allows legal suits to be filed for medical care, you may be beyond the time that you could file a suit. In the US, for example, you get about 2 years to file a suit. It is a very painful process and most people regret the strain of the arguing when they could get on with their lives. The legal person usually gets all the money in the end or a countersuit may be filed that says you smoked, for example, so you should not get such a settlement, etc. Fight instead for advocacy and medical review. It will help the anger a bit.
By getting a clear idea about what your limits and your abilities are and getting a plan for how to do things that you must do or that you would enjoy to do, and by having people around you who understand and care, you will have less stress and you will feel much better. Look forward, not back.
Do not resist counseling or therapy if you are not able to redirect your emotions to a useful state (away from hopelessness, helplessness, anger and toward helping yourself, being hopeful that you can do things and having a purpose). Sometimes you just need a few close friends who can tell you to stop chewing on the tough past and start savoring what you have before you. Get nutrition counseling for diabetes. We learn new things all the time about how to approach diabetes.
I hope this did not offend you and I am sure it provides help for many who never ask questions. Thank you for helping the silent many.