Is it legal for a U.S.resident to open an stock account in Hong Kong and trade stocks listed on HKEX?

The world's financial markets are now globalized.Any residents can open an online trading account of that particular country by fulfilling the required criteria.

Absolutely

Yes, you can open an account anywhere in the world and trade on local exchanges. The US govt likes to make it difficult, and some investment firms do not want to do business with "US persons" because of US govt regulations, but it is perfectly legal and legitimate to do biz anywhere in the world. If you are a US person, you are - of course - subject to US income taxes on all income from every source worldwide.

US person = US citizen wherever you live, US legal permanent resident (green card holder) whether you are living in US or not, any person living in the US (e.g. employment visa holder, student visa holder, illegal alien, or anyone living in the US temporarily or permanently whether legally or illegally present). Furthermore, anyone who has been in the US for any total of 8 years out of the last 15 years is subject to exit tax when they depart the US or they may remain subject to US income taxes for up to 10 years. You definitely want to diversify your assets and hold some outside the US, regardless of citizenship or residency.

yes you can

You really do not have to open an account in Hong Kong to trade stocks there. There are some US brokerage companies that will trade there for you. Two that come to mind are Fidelity and Interactive Brokers. I am sure that many of the larger full service brokers will also handle those trades for you but at a considerably higher cost.

You must also obtain a Taxpayer ID number and file a Hong Kong tax return....any gains you make will be potentially taxable by both nations...

Yes, you just need to claim everything on your taxes. Again, you can have Swiss bank accounts as long as you claim them.