If the buyer in a subject to deal keeps up their end of the agreement paying the seller diligently, is there anyway to safeguard against the seller not making the mortgage payments? 

While I have all the confidence in the world in Civil Engineers to be able to solve a problem like this, I also have equal or greater confidence that a landscape architect would be better suited, as many of us have years of experience working with residential home owners, and a greater sensitivity and eye for aesthetics that are often overlooked by our civil counterparts. 

Secondly, If you are worried about the safeguarding of your trust assets, hire third party, professional trustee and administrators.

Are there any remedies or proactive measures the homeowner should take to safeguard his interest?

They told me they aren't technically open for sale yet but I could use the code EARLY (cap sensitive) to get them! 

She is chemically sensitive from pesticide exposure, and they had a major fire in their home a couple of months ago.

They are already wealthy to an extent and some make 7 figures a year income and on the low side maybe 300k a year profit from job or business.I don't know your situation but it sounds like you are sensitive to max yield earning potential off of the 900k investment.

Off-market multi acquisition is time-sensitive, but also you can't rush it or they come to market like you said - low rents and high price.

As a landlord, safeguarding your investment and upholding the integrity of your rental agreements assumes paramount importance.

Realizing the importance of safeguarding ourselves in a potentially volatile market, I knew we needed to focus on homes with house-hacking potential.