✔ 最佳答案
1. The major difference is (at the least in Hong Kong internally) HKSAR Passport is a proof of citizenship and HKID is not such proof. But both are proof of residence.
Also - HKSAR Passport is a travel document. But HKID is not (even through you can use it going to Macau).
2. There is no visa-free agreement between the U.S. and Hong Kong. So you definitely need a visa.
Anyone can own a house in the U.S. So as soon as you are able to go to the U.S., you will be allowed to buy one.
Unless you hold an immigrant visa, there is no way you can work as you want (even employment visas are available, but they are restictive).
3. Get married with a U.S. citizen, including same-sex marriage.
4. Permanent Resident Card (known as Green Card) is not only a proof of Permanent Residence, but also a proof as an immigrant.
Simply saying - Permanent Resident is a fancy word for immigrant (which you are incorrect).
A Permanent Resident can work and live the U.S. without restriction.