What would be the only instance where RESPA would apply in this case?
There is not an "only" instance. If your loan was structured where the loan was amortizing and upon sale of the existing home the balance would be paid down and then modified, the original loan would be subject to RESPA IMO.
Would the loan have to have a maturity date of two years or greater or be amoritizing for RESPA to apply?
As I've said beofore, don't get hung up on the 2 years. The 2 years applies to construction loans.....Any construction loan for new or rehabilitated 1- to 4-family residential property, other than a loan to a bona fide builder (a person who regularly constructs 1- to 4- family residential structures for sale or lease), is subject to this part if its term is for two years or more.....