My 2 cents.Kids need to be taught how to manage money and wealth. 

Here are some tips.#2  MINDSET:  Just like some people will NEVER become real estate investors regardless of time, money, related profession, - MOST people will never become real estate investors.   

I work a lot with investors outside of the US from a tax perspective but from a mortgage perspective, as mentioned above hard money or DSCR is probably your best bet.

If you don't have to own your own residence though, why not invest the money in a profitable investment elsewhere and rent in Japan?

The new PM did rent the property for more money.

The ADU would be utilized as a short term vacation rental. 

The truth is, the money is still owed, nothing was saved or earned, and you are just throwing money away on interest.Here's a guide that describes what good cash flow looks like and how to analyze a property.https://www.biggerpockets.com/blog/rental-property-cash-flow...

Then keep the speed of money & time moving to make more money in the end.

Any REI worth anything, can invest in any "general RE Market", and make money. 

As you may have found in your search, there are lenders that will lend up to 80% of the appraised value on investment properties and don't require an appraisal.Let me know if you'd like me to point you in the right direction.I have not; I'm still working on acquiring my next house hack, but when I need it, I'll probably use a combo of hard money and margin loan.