@Joshua Lanzieri when renovating older properties there are almost always negative surprises.Go watch any HGTV renovation show and most of them will have this as a cliff-hanger, forcing the choice of either going over budget or cutting back elsewhere.10% overruns happen often, but 50% is NOT acceptable!

Be patient and don't force deals. 

Forcing them to honor the lease is going to be a headache for a year.

For me, I am a little hesitant because I would be in a situation where I would be trying to secure traditional lending (which should be very feasible given credit scores & what $$ amount I am pre-approved up to from my banks), meaning that I assume I would be forced into using my first-time homebuyer loan incentives.

Their clear incentive is to bet on falling interest rates between now and debt maturity.A small handful of owners are forced to sell due to a looming balloon payment that a refi won't cover, but you're not going to find one of those by driving the market when any commercial broker can just look up properties on that basis and start making calls. 

If you are very conservative purchase a fixer upper so you can force equity and hopefully have a low enough payment so that you can LTR the property or even sell for profit should the STR avenue not work. 

If your property manager is not listed as an additional insured, they may not be covered under your policy in the event of an incident on your property, leaving both of you potentially exposed to liability.It can save you money.If your property manager is not added to your insurance policy as an additional insured, they may be forced to purchase their insurance policy to protect themselves.

The numbers no longer look nearly as good on these investments (now more like speculation) and I will now be forced to get my deposit back or accept this new price.

It’s still a worthy strategy to force cash flow that’s rare to find in SoCal, especially if you can reduce the cost of construction by doing a conversion/using an ADU grant or even buying a property with an existing ADU In general, even if the ADU valuation matches your cost including grants, they are usually not worth it in southern CA in single family zones. 

This forces prices down long term.