The seller had a military contractor job with security clearance.

I would check this out with a seasoned securities attorney for their opinion/guidance.  

Consider all the things that could go wrong and see if the lease addresses them: unauthorized pets or tenants, early termination, security deposit, lease violations, late rent, eviction, lawn maintenance, parking, etc.5.

Additionally, a lender deposit ranging from $20,000 to $100,000 is necessary, along with a rate lock-in fee that typically amounts to 2% of the loan amountIn the process of raising equity, it's crucial to be diligent while filling out necessary forms and securing required verifications.

It is widely recognized as a loss leader within the industry and those who are successful normally have a complimentary business such as a brokerage or a construction company that performs complimentary services.

Sure, but again securing $500,000 isn't difficult.That said, liability coverage is lawsuit protection coverage just like it is for you. 

It depends on a lot of factors; primarily the local jurisdiction rules that may limit lease changes (in Portland/Oregon rent, security deposit, screening fees, an no-cause evictions are heavily regulated).

With your point, it seems like living security of 'where's next' for them will be the biggest obstacle for the seller to overcome.

You are not a borrower, you are not liable for the debt, you may need to sign the security instrument to allow the mortgage to be recorded, but this does not change the above statements.

When its included in a lease for real estate then the laws pertaining to residential rental security deposits in your state apply but when its separate you should have more leeway to get some additional deposit to protect you.