Can a daily routine (years) of excessive straining on the bowel cause a loss of peristalsis in the colon? what is the chance of diverticulitis?

NO: The straining won't cause loss of normal gut motility, but it could cause anorectal problems, such as hemorrhoids. I suggest you focus on a good, fibre rich diet, good hydration, and add a bulk laxative to your daily regimen. These steps will not only help your bowel function, thus reducing your need to strain, but will also reduce your risk of diverticular disease. Good luck.