Retiring Overseas for the Adventurous Spirit

I was reading the yahoo finance article "Retiring Overseas for the Adventurous Spirit"

They say

The tax regime for high-net-worth expatriates who renounce their citizenship is even more complicated. The IRS requires these individuals to continue paying taxes for up to 10 years after renouncing citizenship (and leaving the country) unless qualified for one of two very narrow exemptions pertaining to dual nationality or age.

But the instructions for 1040-NR

only say that that you are subject to taxon your US source gross income and gains. So capital gains anddividends on foreign companies (and I imagine foreign companies listedon the American exchanges) are not taxed, nor is interest from foreignbanks taxed. Also, when your AGI excludes your foreign income, yourindividual US tax rate is less. Generally, for non-citizens and non-residents, interest from US banks, interest from treasuries, capital gains on US stocks are not taxed But forthe expatriate under the 10 year rule, these items are taxed, eitherat the larger of 30% or individual rate, or the individual rate. If you have long term capital gains of 1M, the individual rate for US citizens would be 15%, so for an expatriate under the 10 year rule, would their tax be 15% or 30%? It seems to me to be 15% as an ordinary non-citizen would not pay tax on this capital gain, so for the expatriate under the 10 year rule the tax would be the larger of

15% of 1M or $0, which is 15%.

If you have 100k in treasury interest, then by the same reasoning I think that the tax rate would be the individual tax rate (which is anywhere between 0% and 35%). However, by excluding your worldwide income from your AGI, your individual rate is lower.

They also mention

Passed in May, the Heroes Earning Assistance and Tax Relief Act, or HEART Act, goes a step further and imposes income tax on any net unrealized gain from a property sale in excess of $600,000.

What on earth is unrealized gain on property sale?

In any case, you always have to pay US state and federal taxes on your realized gains after a sale.

Finally, they say

Fortunately, the U.S. has tax treaties with a number of countries that either reduce the tax burden for U.S. citizens or provide exemptions from paying foreign taxes.

They should say that the treaties most often prevent double taxation.

Finally, are non-citizens and non-residents still allowed to receive social security checks? On the social security website I did not find anything to contradict this, so I imagine it is allowed. And if so, I imagine it would be taxed on form 1040-NR at the individual rates?