For 2 and 3, no capital gain taxes in the US, and the witholding tax depends on your country of residence and its tax treaty with the US. I've experienced from 10 to 30%
With IB you don't need to declare anything to the US tax authorities AFAIK, IB will withold the dividends at the proper rate, and if you think something is wrong with the rate, you should contact IB first.
This is if your paperwork is in good order to trade, but IB will lock you of your account management until everything is in order, than will lock you out again when they need an update.
1256 ES future is a lot of money

Thanks for the info! So to summarize:
Do I have to fill any document in any of the cases above and send it to the IRS? Or does IB do all the necessary paperwork?
- Nothing to declare to the IRS (US tax authority) about any capital gains obtained by trading US stocks, ETFs, futures or options.
- If I ever receive any dividends, there is a withholding tax and it'll be deducted directly by IB. The amount depends on the tax treaty between countries.
- This is supposing the paperwork is all done with IB, including proper filling of the W-8BEN form.

I always recommend you check with your local tax authority/accountant if in doubt but:
1) The US does not tax non-resident aliens living abroad
(If you spend enough time in the US to be classified as Tax Resident than you are viable. For the US this is I believe about 6 months).
2) Income you generate from capital gains will be subject to taxation in the region that you are tax liable to. If you live in the EU you'll be taxed by them on all your global income.
3) Dividends are collected by the US at the rate of 30%, however you may be able to reclaim some of this if you have a tax treaty. If you are registered with IB as being in a tax-treatied area with favourable tax conditions I think this gets reduced to 15% for the EU.
Exactly, and there's no need to send any document to the IRS.
I always recommend you check with your local tax authority/accountant if in doubt but:
1) The US does not tax non-resident aliens living abroad
(If you spend enough time in the US to be classified as Tax Resident than you are viable. For the US this is I believe about 6 months).
2) Income you generate from capital gains will be subject to taxation in the region that you are tax liable to. If you live in the EU you'll be taxed by them on all your global income.
3) Dividends are collected by the US at the rate of 30%, however you may be able to reclaim some of this if you have a tax treaty. If you are registered with IB as being in a tax-treatied area with favourable tax conditions I think this gets reduced to 15% for the EU.