Will this be considered a repair (as I didn’t replace all windows, only ones that didn’t function) or improvement?

After all, even $150 savings per month is equivalent to the cash flow from a single rental unit.One thought that crosses my mind is that I probably wouldn't make insurance claims on small things like appliance replacements, since it will increase the policy premium(?) 

I’ve got a tenant who wants to end her lease early and she found a new couple to replace her so I have agreed to this pending their approval of my screening process.

Keep their security deposit and handle it according to your state's laws and replace, cleanup, and screen a new tenant. 

I am a licensed builder and have replaced hundreds of roofs and I would never even consider being responsible for ordering all of the materials that are needed.  

If I die unexpectedly, my family is taken care of.Retirement accounts were meant to replace company pensions by reducing employee income during employee working years to save businesses money.

Leave for tenants- If I leave for tenants I would note it in the lease that these are the tenants responsibility and once broken I will not be responsible for fixing and/or replacing.

You might give some contractors a call and ask a general questions such as "What is the average cost to replace a two story house roof in XXX area?"

We are having constant a/c unit replacements in my 15 unit multifamily . 

However, we've explored using some corrugated metal fencing for the next ones we need to replace, something like this.