A little different fact pattern. According to the description it looks like that property has a well and septic, too. Being newer shows that the county was still allowing 20 years after the OPs home was developed.
With this larger/newer property it looks like the topo includes a level terrace above the flood plain area in the rear, with pics in the listing showing retaining walls for about 40ft behind the structure. The rest of the parcel extends into the flood plain and all the way into the centerline of the creek itself. So maybe the septic install is located further from the structure, or maybe they installed it in the front.
Besides, the question we'd want to ask is about the financing itself. I don't know how anyone does that in TX without calling brokers and asking. There are properties located on both sides of that waterway. A 5yr check on the listing history would reveal whether brokers were warning about septic systems or not. That's how a local appraiser would normally get clued into asking about those specifics, if they're seeing it being mentioned in the MLS listings.
If an atty can demonstrate that FHA was or wasn't adhering to the location element of their MPR then there would be no need to assume what they would have done in this situation. For all we know, when FHA responded it wasn't their problem that may be the result of them knowing from their own conduct in the past in that area that they weren't enforcing that element of their own policy. Or not - maybe they have been scrupulous about their MPRs in this situation and their position is that the borrower had no right to assume FHA would have protected her from her own decision making.