Additionally, every property is unique with different methods and terms of financing, performance, tax consequences, etc...

Consequently new construction is almost exclusivley limited to 450 to 650k homes.

For example, if you sell a property for $1mm and receive $900k in cash and carry a second mortgage for the final $100k how does this affect the 1031 and tax consequences?

SFHs, not being purpose built rentals, can easily be sold without any negative consequences.

Elderly owner of 20 unit complex selling portfolio but suffered significant tax consequences ( assume capital gains) when sold her previous property. 

There are no tax consequences when you do so.Then you can convert your Traditional IRA into Roth, and move to self-directed IRA.

If they can't handle the home, for whatever reason, they can break the lease and suffer the consequences.

Of course, since the payback is variable, perhaps the investor holds a tax consequence, even if I pay the capital gain / ordinary income tax.Has anyone experienced that?

If you have a written policy in place that the tenant is responsible for giving their keys back within X amount of time after move out, and have said the consequences for not doing so are paying for the re-keying of the property, then yes, you can take that out of her deposit.

It was specifically to help a CA investor make an informed choice about investing and highlight the potential tax consequences of his decision.