I did not mean to imply a junior was ever forced to agree to a short sale...just that the amounts to which they will agree are usually very predictable. 

Predictably the answers are all over the place, since people's skills and talents are greatly variable.

By doing this you are creating a very low risk cash flow instrument. 

Ask yourself questions like: What if we can't raise the rents as fast as we predicted?

Seriously, any instrument representing money is fine.

If this hurricane is as bad as they predict there is going to be a lot of homeless families due to the storm .

Mortgage is an instrument used to secure the note, if you were buying a house, borrowing money for the purchase there would be a mortgage or a Trust Deed securing lender's interest, it is not applicable for the 401k participant loan.

It is impossible to know for sure, because know one can predict cap rates 12 to 18 months out.

There's been a property down the street from me that has been listed a few times, and I've looked at it from the outside (and that was enough to scare me and likely anyone else who has looked at it) which doesn't help me predict rehab cost.

Taken together, they can have predictive value.