$ 98560 (for a full eleven years on the full $ 350K).  I agree - even that amount wouldn't move the needle to $ 134000.
I still want to know about how the allocation to land was handled by either the DIYer, or the auditor

I have no idea if a "retroactive" F 3115 on an amended 2016 is possible (is it @TaxGuyBill that know these things by heart?).
It sure sounds like the existing trail of the cash payments is not bad (signed receipts & F 1099 done).  Either the auditor is being a real PITA or there's more to this story.
You're right - I have something posted at the office about (+/-) "if you thinks it's expensive to hire a tax pro, wait until you see the consequences of doing it yourself".  This appears to be a perfect example of that !