You could build up quite a portfolio by the time your husband is ready to retire without having put anything down. 

The right property owner you are looking for is someone who are looking for long term stable income, don't mind take a little discount and maybe waive the first month rent if you can somehow convince them.

Will they be prepared to address retirements and disposition?

These properties cover my mortgage, allow me to save for maintenance/repairs, and are projected to be worth $7–9 million by retirement.

I am not saying you have to find a way to get a $100k/yr job, although that wouldn't hurt, but a stable W2 salary is going to help a lot. 

This could mean investing in property flips or purchasing out-of-state rentals for more stable income, but it would require letting go of the $1,350 rental income from the manufactured home.What factors should I consider for each option, and how would an experienced investor assess this situation mathematically?

It’s nice to have a million dollar property I didn’t pay for as part of my retirement plan… ;)Makes sense.

People who have money may prefer to seller finance but if someone is retiring they most likely don’t care about a higher sales price they want to avoid the riskYou mention ou have a 100% close rate - then why not just buy it was traditional financing if you really want the deal?

I love being able to turn personal properties into rentals as they required a small amount of cash for a large asset to have someone else paying off.It really depends on your long term goal, but ours is to use these "retirements baskets" into reasons that early retirement at 44 is successful! 

If you are headed into retirement, my way would still be way safer than your plan.