Idea is I may just sell the property the moment the project and renovation is finished.For context, the ADUs will be 2 2bed 2 bath units, 800 sq ft each, 1,600 sq ft total.

There are many many threads in these forums about it, some of the information may be a bit dated, but the general idea is the same.

Any ideas would be great!

Will follow up to get more details on your idea!

I totally understand and acknowledge the idea of investing where you currently live. 

Another idea could be to see if the HVAC system can be adjusted or modified to better separate the airflow between the units.

No, it's not a good idea, but it's possible to do correctly if you don't rush into it.

I like the new construction idea a lot.

I have no idea what you mean in regard to lending to my cousin, but it doesn't matter. 

However I did note that houses are mostly really old, e.g. 100 years or older... any idea if this poses challenges in things like lead paint presence, outdated wiring, rusty plumbing, etc., or have many of the homes already been updated?