Unfortunately, here you have quite a lot going on and maybe misinformed on a few areas.

The time depends on lease length and how long they've been living there.I would also recommend reaching out to their caseworker (if they have one) to let them know the lease will not be renewed.Section 8 voucher holders are incentivized to vacate quickly in order to keep their voucher and avoid having to pay full-market rent out of pocket elsewhere.Hope this helps!

Yes, this is as niche as it gets, so odds are slim, but I'm curious to hear if anyone else lives on some acreage while boarding other horses on your property?

Any ideas on which policy might be the best fit?

Your focus on an arbitrary unit count isn't indicative of success and can lead to bad investment decisions.  

You can see regularly companies and some individuals here on BP that like to play games and it happens to them.   

If you are signing contracts on behalf of others, then you are required to have a real estate license or be an attorney.

I am looking to refinance an existing HELOC to increase the line of credit.

Having to buy another primary residence, to avoid tax on the gain of your current home, has not been a requirement for over 20 years.Assuming it was your primary residence from the beginning, and you have not moved out/rented it for more than 3 years, you gain is exempt up to $250k filing single and $500k married/filing jointly.

@Paul Azad weather the juice is worth the squeeze depends on the specific deal and details.