We would like to learn about how we should have went about it so that we can purchase more properties, minimize mistakes, and reduce our reliance on corporate life.

You never skip interest, you just reduce principal by $600 so you pay less interest.

Some used more creative ways to handle the needs but it is still a capital call and dilutes the return (if anything left). 

If the Seller will not reduce the price to where it still makes sense to you then do not do this deal.

A professional property manager should handle the heavy lifting, ensuring your investment is protected and profitable without adding stress to your plate.

However, you'll likely be able to put more money in your pocket if your reducing your living expenses in a HCOL area.

You could also add cash to your exchange account to reduce the debt replacement if you wanted. 

My father is a certified contractor in NJ—he’s built one home, flipped four, and consistently handles upgrades and maintenance for me and his other clients.I’d appreciate any tips or stories from those who’ve turned equity into a profitable venture.

Your son will be forced to sell or refi, and I bet you that this deal can't handle that situation. 

If you hate the process of placing tenants, it may be worth it to hire a property manager to handle tenant placement for you.I think finding a new tenant is a better long term strategy if you're confident you can find a higher quality tenant that will cause you fewer headaches.