I can only speculate but I suspect she either met someone she's moving in with, or decided she wants to move to a cheaper place, nicer place, or is buying a home.

I just bought a property where my lender gave a mortgage on the purchase with 20% down and a second mortgage on some stuff.

I would suggest you talk to a mortgage broker or a bank though - if those homes are zoned commercial you will probably have a hard time getting an FHA or low down Conventional mortgage.

They also have deadbolts now that you could install on every door which we have contemplated, but it's cheaper to buy lockboxes and only use them for the vacant units than buy a new lock for every single unit we manage. 

Not just because I live there but because it is generally cheaper and better neighborhoods are available.

If you don't have much money, then invest your time and energy.Putting a second mortgage on your home to buy a turnkey property is a horrible idea.

The vacant lot owner can't do anything with that land, so they should be willing to sell it for cheaper than market rate.

We love our home which has a $2,400/month mortgage and we rent the furnished MIL downstairs for $1,600/month, so I plan to do the equity line of credit and continue living in this home.

Our mortgage was significantly higher than our flood insurance, and I don't know if we qualify to try to apply for FEMA or if there is other assistance, and if so, if we would apply as a business (we file it on our personal taxes as rental income).

It is still subject the mortgage rates and business plan to make the investment profitable.