I am new to real estate investing and got into it because of the book The House Hacking Strategy.I live in San Diego; the rent is just crazy.

If you sell a property, and do a 1031 exchange with the proceeds, but don't need all the proceeds for the down payment of the next property, are you taxed on the additional profit that isn't part of the exchange?

For example, I see newer investors attempting wholesaling or bird-dogging for other investors to generate additional income to invest.

I am considering building up on my fourplex. 2 additional units.

These units are perfect for owner occupants, and they are perfect for related living as you have described.

It will be remote, but since you live there now, you probably already have connections who either already know the house or local friends to find you good people to service things as needed.  

We live in Texas, a community property state, and are 50/50 owners.

When your tenant stops paying and communicating, who will knock on the door to see if they still live there?

As for valuation, I see no reasons why a Junior ADU would negatively impact additional housing unit.

However, you have the additional question about if and how you might enforce a judgement in another country.