Eventually, it got to the point where I reduced her role to part time and then had to let her go.

These days, when people sell on land contracts, it's often with the intent of not recording it, trying to reduce the chance the original lien gets called due on sale.

Is there anything I can do to reduce the gain?

However, if your father inherits and later transfers the home, it should qualify for a step-up in basis to fair market value, reducing future tax liability.If the goal is to tap into equity, having the trust take out a HELOC may be a better option, avoiding California Prop 19 reassessment, which could raise property taxes significantly.

This reduces exit options and affects the value. 10) Small number of small units is the most expensive residential development there is.

If you have money for large down payment that can reduce the payment which lowers the income required to qualify. 

In your case going to a fifth unit as noted above could actually reduce the value, so I'd be cautious of that since that materially degrades their collateral.

If you have reduced the risk as much as practical, feel that the potential upside is sufficient for the risk that you will take, and have the ability to bear the risk/survive a bad outcome, then it's time to take a swing.

If flipping will be ongoing, electing S-Corp status for the LLC could reduce self-employment tax by taking a reasonable salary and classifying the rest as distributions.

Using a Whole Life Insurance (WLI) loan avoids banks but accrues interest, potentially reducing policy benefits—compare this with other financing options like HELOC or seller financing.