Given no parameters and assuming I didn't want any more rental property, I would probably pay off the primary and open a HELOC on it, then pump excess cash into a reasonable low-cost S&P/Whole market index fund like Vanguard.

Net root issue was low market confidence and a supply of housing inventory that had excessive deferred maintenance.  

In a downturn, single family tends to loose value but MF doesn't. 

But on the brink of retirement, "excessive" risk is not the right move either.

Here is the exact text of the law:(A) Any security deposit in excess of fifty dollars or one month's periodic rent, whichever is greater, shall bear interest on the excess at the rate of five per cent per annum if the tenant remains in possession of the premises for six months or more, and shall be computed and paid annually by the landlord to the tenant.Instead of charging an extra security deposit, you may want to consider a risk mitigation fee.

Aside from the above info I will say I would not want to be at the leverage point your suggesting; but I'm not excessively aggressive in my investing strategy. 

using the financial health and equity of one property to finance the purchase of another  property.in simple terms, if you have $300k excess equity in a project, you can cash it out and use it as the downpayment for another property.the new property must be able to pay its own mortgage and the cost of the equity cash out.

Overcomplicating things or being excessively involved in the situation from afar (when that is what you’ve hired the property manager to do) always makes me wish I had just paid the quoted price on the front end. 

I stuck with SFH and followed a loose BRRRR strategy.

If you analyze the situation you may have way more to loose by not helping your parents out.