I have started using independent septic inspectors who just inspect the system, not companies who build septics, pump septics, etc because then they have an incentive to tell you that you need to buy their product.

One option you may consider is taking down the property with private money or short term financing then when the property has been operating successfully for a period of time refinance into a 30 year fixed product to get the best interest rate and terms you can find.

You never know until you ask:) There are ones out there that have special, temporary products that will do it, but it's extremely rare, usually temporary, and used as a promotional product...and then it disappears. 

Blood from a stone..Try a Fluidmaster with the chain bobber thingy. https://www.fluidmaster.com/pro-series-products/flush-valves/pro57k-flush-valve-kit/Also look at their Flush Valve Repair Kit.

May I ask why you are opting for the FHA OTC instead of a Conventional OTC product?

This will make your offers much more competitive and is an easier product to find right now than 203k.BTW this advice also goes for conventional rehab loans. 

I guess the difference is I am risking millions and producing a product and doing a ton of work to create the delta. 

There is a company called crawlspace ninja that sells a higher quality product (i have no association with the company). 

But if I can streamline this up front effort I feel I can be more productive in closing deals and either fixing and flipping or BRRRing.

So that is a fundamental part in the feasibility process to understand how much actual product you can fit in your development.