It is the cause of the problem and will continue to be troublesome.It is just going to cause more issues down the road--like eventually crack your next driveway, impinge on any underground pipes, put leaves and branches on your roof/gutter every storm and every fall, etc. 

I have taken and Airplane   a Mercedes and a Boat  few other odd items as trade for real estate .. with the airplane being the best one.. but its hard to get them to do that because most that want cash and all I had to trade was vacant building parcels.. so with cash flow that might be different but they would have to come up with cash to settle your mortgage or put a loan on it to take your mortgage out.. you don't want to sell sub too in that instance as its fraught with seller risk and it will impinged your ability to get new financing.. 

Keep in mind you are going to have to live there so make sure that it doesn't impinge on your personal lifestyle. 

The old growth trees which appear to be impinging on the house could be viewed as a negative in terms of potential damage to the house in a wind storm and to the foundation / basement over time.How does this house's square footage stack up against the comp.'

The reason given by the Washington Legislature forbidding non-owners from paying someone else's taxes is that some owners were being scammed by investors paying the owner far less for a property he/she was about to loose, then the property owner would have received in excess proceeds from the highly competitive property tax foreclosure sale.A number of local investors say that the real reason for the change in who can pay property taxes, is that many county budgets were being seriously impinged by investors removing excess proceed funds that otherwise escheat to the county general fund.

Would this impinge on their renters rights?

You would be surprised how often people's "basic liberties" are impinged by a preliminary injunction or preliminary order.

They are impinging on my freedom at that point.

Because as soon as you refi with an institution, it is going to appear on your credit score, impinging on your ability to do further deals.Some icing on the cake, depending on how well you know the couple, and how old they are - You could ask them to take out a life policy on their lives, that will cover the capital on the loans should they die - The surviving spouse is guaranteed of getting the capital back, and you will have a windfall that can be used as a nice deposit for something else.

But, many realtors feel that impinges on their space.