I'm selling a property for $49K with Tenant in $825/mo with ROI=14% PM included.

I was told single person LLC is most popular and you could include your spouse. 

You definitely have options, including a HELOC on your primary to free up funds for investing.

The definition of “bad deal” here would relate more to the opportunity cost of such investment compared to other investments (financial markets), than a real financial loss for the investor (i.e. ending up with less cash than originally invested).Let’s now modify a bit our assumptions by including some inflation in our model (say 2%, in line with historical average).

This doesn't relate to the title of the post, but I was wondering if there are any specific terms that are a good idea to include when lending for a flip.

Do your current expenses include saving for repairs, maintenance, capex, etc.? 

Unlikely it will sell at the auction, unless someone bids the full amount owed including of course all accumulated interest.  

I have considered any and every expense associated with the project, including credit card/paypal fee's, credit card interest payments, travel expenses.

Operating expenses include water ($480/month), insurance ($150/month), trash ($60/month), and property tax ($256).We'd like to offer asking price at 6% amortized over 30 years with a balloon at 5 years.

It a very detailed pattern and even includes folds!