Can. You need to think about how to fund and withdraw money.
Nope. Just make sure don't close your bank account when you lesve UK.
Of course. You cannot run away from dividend tax unl SS your country got treaty with Us govt.
Unfortunately Malaysia doesn't have. Hence you can I'll be taxed the full 30% if you buy US stocks or etf.

You could use Bigpay instead of a CC. That way you get the best exchange rates in and out. Of course the limit is the issue but with no withdrawal fee, unlike etoro's $5, it's not really a big deal since you can withdraw up to your limit and then move the cash out. Just rinse and repeat.
Not worth holding US dividend stocks if you're not an American or one of those countries with a tax treaty. But then again even with 30%, it's still better than FD 

I don't like the their web platform with line chart.  Do you know how to remove the chart info?