Just reduce there rent by $5 and they will be happy to do it.

It might be a situation where you are digging a manageable hole that you can work and earn your way out of, but be honest with yourself.

After five years, I am willing to buy the other owner out or they could stay in and continue earning and enjoying the property. 

That can be used to buy down the mortgage rate or reduce closing costs. 

Even in the nice locations, most top out around $2,800.Of course there is also your increased insurance, vacancy, repairs, CapEx, and the never mentioned reduced rent on the main house since having a stranger living in your backyard usually isn't appealing to most renters. 

House hacking with an SFDU could be a smart first step since it allows you to reduce your housing costs while building equity.

Normally when a  property is advertised as "cash only" the property is in poor condition and/or the seller wants to reduce contingencies and opts for a cleaner transaction.

As for your neighbors, they could have exemptions like Homestead, widow, military disability and many others that can reduce the amount of taxes.

Unfortunately they don't care how much your worth they care how much you earn a month.

Yes, Hong Kong certainly is an interesting place, a good place to earn money as well.