Personally when I rented I was always rolling my eyes when I saw a coin-op machine in the building and felt like that was just an extra hidden cost into my rent.

I've seen some do an acid wash and not try to cover up the existing but just clean it up.In the photo, the real wall paint color is by Sherwin Williams Interactive Cream.

Will my efforts amount to a wash, or will I be able to just take their exam and keep moving?

The days you are not busy doing real estate stuff you should plan on being at a kids ball game, a pancake breakfast at the local Lions club, a school play that you donated $50 for, getting your car washed at a local fundraiser by the soccer team, volunteering at a local chicken BBQ, taking your dog to agility classes and meeting people etc etc.

It sounds like a wash except the $12k is a one time cost that is below the line and does not affect the NOI.

Problem was he didn't even show up.

The house also needs freshening up (it's a time machine to 1982).

The benefit(s) of diversification will likely get washed away in the cost of management. 

It took me about 1 hour and I was done . now I have lots of time to get a replacement the spare covers the current  problem .I dont repair refrigerators , washing machines or dryers , its cheaper and faster just to replace .

Odoban, as well as an ozone machine, may help.