First you must file eviction papers.

It also has loose/damaged siding, security bars on windows that need to be removed, earth-to-wood contact in the foundation and siding, leaks in sinks, windows that don't stay open, loose toilets.

I would follow through with filing for eviction because they may not move out at the end of the lease.

Since this real estate business would be owned by my mother and us three boys, I am unsure what legal entity would be best to setup for all of this to achieve the most legal security and tax advantages down the road. 

I have raised capital in the past from different investors securing funds in different ways.

Howeve, You may want to take a small salary to make sure you can get social security and Medicare if it’s still around when you are eligible.

Are you aware that the country is living through an important security crisis since about 3 years ago?

You simply file a complaint with the commission , he has a bond posted and you can collect from that .

Flipping by nature  most likely will be considered a business holding in renting give you the luxury of not paying Social Security and Medicare.  

In my limited experience the better tenants don’t wait til right before they need to move to look, they look and want to secure a place to live a month or so out, or at least a few weeks out.