I'd rather not commit mortgage fraud but that's just me. 

This has the potential to go down a rabbit hole where they can ask for invoices for personal loan's utilization and if you alter those documents, you WOULD be committing mortgage fraud

That's where the mortgage fraud issue can come into play I've heard however. 

Look for markets that cash flow and fit your investor identity. 

Would it be legal for us to "technically live" in the attached apartment as our primary residence (send mail, bills, leave some belongings) and then rent the main house or would this be mortgage fraud? 

It's mortgage fraud and a federal offense.  

the only way around that would be to not tell your insurance company, but fraud lays you wide open to all sorts of bad things.There are companies that won't insure roofs older than 15 years.There are insurance companies that won't do STR at all.  

if the bank investigates AND they can prove you knew about the job change before closing they can pursue you for mortgage fraud. if the job change legitimately came up after closing, you are in the clear.

If comparable sales in the last year were $115,000, but there are five properties almost identical to yours, in the same neighborhood, listed on MLS for $100,000, then your value is some place south of $100,000.

If you don't state up front you are going to use it as a commercial venture, that is mortgage fraud.