Regarding the 60k estate tax thingy, so if I open an IB account, I can't have more than 60k cash in the account at any time (including certain taxable assets like AAPL shares)? At any time means 1 second, 1 day ...?
You certainly can! You can have $1 million of cash in your IB account if you wish, and if you're able.
If you're a non-U.S. person then, at the moment you die, your cash (in any currencies) held at IB counts toward your estate's US$60,000 estate tax exemption. So would your shares of Apple (AAPL), whether those shares are held at IB or via any other broker. The only tax difference is that, at a U.S. broker, your cash (in any currencies) counts toward the exemption. If the cash is at a non-U.S. broker, or even at a U.S. bank, it doesn't count -- it's not U.S. estate taxable.
If you're concerned about that, then there are two basic techniques at IB (or at another U.S. broker) to reduce the exposure:
1. Don't keep much cash in the account for very long. One way to deploy idle cash is into short-term U.S. T-Bills, which are available for purchase via IB.
2. Buy some more life insurance to compensate for the estate tax.
There's also a third technique that some can use:
3. If your heir is your legal spouse (same or opposite sex), and if your spouse is either a U.S. citizen or quickly becomes a U.S. citizen within 9 months of your death, then (I believe) the U.S. estate tax exemption rises to infinity -- the unlimited spousal exemption applies. This only works if the (sole) heir is a U.S. citizen. It doesn't work quite as well in the other direction, although a U.S. citizen who dies has a much higher estate tax exemption (US$11.2 million for deaths in 2018). And I think this is a citizen-specific privilege, but maybe it also applies to U.S. persons more generally.
Yes, there really are some non-U.S. citizen widows (usually widows) who are married to extremely wealthy American men (usually men), and they
quickly naturalize as U.S. citizens within 9 months of the death (if they're able) in order to reduce the tax rate on the estate they stand to inherit to zero. This really happens, on occasion, and it's perfectly legal.